Pigment Metabolism

PIGMENT METABOLISM. Under P. m. are usually understood all the processes of formation, transformation, and breakdown of blood pigment (hemoglobin), more precisely its non-protein pigment part, and the main derivative of this pigment - bile pigment (bilirubin). At present, however, other pigments are also known, which by chemical composition are apparently close to Hb - these are myoglobin, cytochromes, Warburg's respiratory enzyme, and other still very little studied pigments. It is impossible to separate the processes of formation, transformation, and breakdown of these pigments from the processes of Hb metabolism. In a broader sense, P. m. can be understood as the processes of formation, transformation, and breakdown of all body pigments, i.e., both the above-mentioned pigments of the Hb group, and all other pigments - melanin, lipochromes, etc. The study of blood pigment metabolism and related pigments presents completely exceptional interest, since these pigments play the most important role in the animal organism - in respiration. Physiology of Hb group pigment metabolism. The main mass of hemoglobin (see) in higher animals is in erythrocytes. But in addition, Hb or a substance similar to it in structure is contained in significant amounts also in muscles. According to Whipple, its content in muscles reaches 300-1,000 mg%, taking the weight of muscles in an adult = 25-40% of body weight, we get 100-200 g of muscle Hb. Thus, muscle Hb constitutes 10-20% of all Hb in the body. The content of Hb in muscles is not constant and fluctuates depending on their functions. Trained muscles contain more of it than untrained ones. When the nerves innervating the muscle are severed, its Hb content rapidly decreases (Whipple). In addition to Hb, the pigment cytochrome is widely distributed in animal and plant tissues. Mac Munn believes that it is close to hemochromogen. The physiological role of cytochrome is apparently related to oxidation processes in tissues. In addition to cytochrome, the so-called respiratory enzyme (see Hemoglobin - hemoglobin as a respiratory enzyme) is contained in the body's cells. According to Warburg, this substance is close in structure to hematin and acts as a catalyst for oxidation processes in cells. In addition to the substances of the Hb group listed above, various non-iron pigments - porphyrins (see) - are encountered in the animal organism and its secretions under physiological conditions. The study of the pigment particle of Hb metabolism, i.e., for clarifying the sources and paths of its formation, the paths of its transformation and destruction, and the fate of its breakdown products, presents great difficulties due to the fact that only a relatively small part of Hb undergoes breakdown to products excreted by the body, and only a corresponding small part of it is built from new materials. The largest part of various Hb breakdown products obtained at successive stages of this breakdown is used by the body to build new Hb. Thus, we have, as it were, a whole series of branches leading by a roundabout way back to the main path in the metabolism of the Hb pigment particle. Determination of these roundabout paths naturally presents the greatest difficulties, all the more so as there is reason to assume that the paths of blood Hb metabolism, muscle Hb, cytochrome, Warburg's respiratory enzyme, etc., do not go separately, independently of each other, but converge together and again diverge at various stages of metabolism. True, in quantity, the Hb pigment particle occupies a dominant position; of other related pigments, at least muscle Hb is present in the body in such quantities that its metabolism must be taken into account in the quantitative study of this pigment group. As for the amount in the body of other pigments of this group - cytochrome, Warburg's respiratory enzyme, etc. - we have no data whatsoever. The first attempt to give a scheme of Hb pigment particle metabolism was made by Whipple. A more complete scheme was compiled by Borst and Konigsdorffer. To build the Hb pigment particle anew, the body can use both materials obtained from food and breakdown products of proteins and fats of its own tissues. Among these materials there are such from which the body can build pyrrole rings anew, necessary for the formation of the pigment particle nucleus. Such materials, from which pyrrole rings are easily obtained in vitro, can be acetoacetic acid, aminocrotonic acid, 1,4 diketones, pentoses, and glutamic acid. But in addition, ready pyrrole rings can also be present in food materials and tissue breakdown products of the body, for example in amino acids - tryptophan and oxyproline. Finally, ready porphyrins and porphyrin complexes are undoubtedly introduced in significant amounts with food, for example in plant food in chlorophyll, in animal food in Hb, myoglobin, cytochromes, and in others. Perhaps during digestion, absorption, and in the organs where they arrive, they first undergo breakdown to one degree or another, and only then their breakdown products are used to one degree or another for building new Hb. Where all these synthetic processes and processes of preparing materials for synthesis occur is also unknown with certainty. The synthesis of blood Hb in its final phases undoubtedly occurs in bone marrow tissue - in erythroblasts; the localization of preparatory processes can at least be partially assumed in the spleen, liver, bone marrow, in their reticulo-endothelial cells, since in them as if the processes of erythrocyte breakdown, accumulation of Fe, transformation of Hb into bilirubin, etc., occur, but it is impossible to more precisely determine the localization and chemistry of all these processes. It seems probable that the process of formation of the Hb pigment particle goes through porphyrin, but it was possible to prove the presence of porphyrins in erythroblastic tissue in amounts greater than in all other tissues of the body under physiological conditions only during the period of embryonic life, and in the adult organism - only during pathological blood formation, as it occurs in malignant anemia, when blood formation returns to the embryonic, so-called megaloblastic type. In embryos, Borst and Konigsdorffer, studying luminescence, fluorescence, and spectra of corresponding tissues, could observe in erythroblasts not only the presence of porphyrin, but also its gradual replacement by Hb, resp. its gradual transition into Hb. In the erythroblastic tissue of the adult organism, the same authors could not prove the presence of typical porphyrins, but obtained results that allow one to assume that Hb formation goes through a phase of a pyrrole complex close to porphyrin. The picture of the reverse process - Hb breakdown - is somewhat clearer. We encounter certain breakdown products of Hb - the bile pigment bilirubin and Fe. Intermediate transformation products between the Hb pigment particle and bilirubin are unknown, but between the degree of hemolysis and the amount of bilirubin produced in the body, a complete parallelism has been established. Outside the animal organism, it has not yet been possible to obtain bilirubin from Hb. One can assume that Hb breakdown begins with the cleavage of globin, then Fe is cleaved off, and its porphyrin complex is transformed into bilirubin. However, Fischer (H. Fischer) considers it more probable that Fe is not cleaved off at once, that the entire pigment part first changes, undergoing oxidation, and that only at later stages of transformation Fe is cleaved off. These processes probably occur in the reticulo-endothelial system, in particular in the ret.-end. cells of the spleen and liver (see Hemolysis, hematolysis - hemolysis in vivo). Already with regard to the intermediate products obtained in the transformation of the pigment particle into bilirubin, it is necessary to raise the question of their use by the body for building new Hb. The same question must also be raised with regard to bilirubin. The use of intermediate products and bilirubin itself for building new Hb can, at least in preliminary phases, occur immediately after its formation in the ret.-end. system of the liver, spleen, and bone marrow; bilirubin can also be used after its excretion with bile into the intestine and reabsorption into the blood from the upper parts of the intestine before transformation into urobilinogen. To what extent such reverse transformation of bilirubin into the Hb pigment particle actually takes place in the animal organism is unknown. Bilirubin remaining in the intestine, under the influence of bacteria, is completely transformed into urobilinogen; one part of this urobilinogen is excreted from the intestine with feces, another part is probably destroyed, but a third significant part of it is at least absorbed from the intestine into the blood, and this absorbed urobilinogen almost completely enters the veins of the portal system. Urobilinogen, entering the hepatic capillaries with portal blood, under normal conditions is almost not passed by the liver into the inferior vena cava, resp. the general circulation.

Penetrating there in small amounts, urobilinogen, along with urobilinogen that penetrates there bypassing the liver from the lower parts of the large intestine (through the inferior hemorrhoidal veins), is excreted by the kidneys and gives that urobilinuria—about 1-2 mg (according to the more precise technique of Teg-wen), which is observed in normal conditions. The fate of urobilinogen that enters the liver capillaries with the portal blood is not entirely clear. The amount of it in any case is significant. If on average about 120 mg of urobilinogen is excreted daily with feces, then presumably the same amount is absorbed from the intestine and enters the liver. As for the fate of this urobilinogen that entered the liver, at least four possibilities should be considered: 1) its excretion with bile as such, 2) its transformation back into bilirubin, 3) its use as material for building new Hb, 4) its destruction with the excretion of its breakdown products from the body or with their use in other directions. To what extent this excretion, transformation, utilization or destruction of urobilinogen actually takes place in the body is still a matter of dispute. Excretion in the form of urobilinogen with bile is doubtful, since the presence of urobilinogen in bile cannot yet be considered established (the technique used for this purpose is not suitable). Its transformation into bilirubin is more probable, but the few existing experiments that speak in favor of such a transformation are still insufficient to consider it established. As for the use of urobilin for building new Hb, there are no direct data in favor of this assumption. The same can be said about the destruction of urobilinogen in the liver. In various pathological conditions of the liver, urobilinogen passes through it into the general circulation and then a significant part of it is excreted in the urine.* The ratio between the amount of urobilinogen in urine and feces then reaches 1:1 and more. This circumstance gives an idea of what a significant part of urobilinogen is absorbed even under normal conditions from the intestine. Relatively recently, a certain connection with the hemoglobin-pigment metabolism of the pigments of urine—uroerythrin and urochrome B (Heil-meyer), the chemical nature of which however has not yet been clarified, has been established; the amount of these two pigments changes directly in proportion to the degree of Hb breakdown; it is for example sharply increased in malignant anemia and especially in hemolytic jaundice, and after significant bloodlettings it is strongly decreased. The liver undoubtedly participates in the formation and excretion of them—every lesion of it is accompanied by a strong increase in the amount of these pigments in the urine. From what has been stated, there is reason to assume that the products of transformation and breakdown of the pigment part of Hb are used for building new Hb. How great is that part of Hb which is formed from these products, how great is the part produced anew from materials coming from outside, this question is partly answered by the experimental observations of Mc Master, Rous; from these observations, as well as from the data of other authors (Singer, Heilmeyer, Otto), it follows¹, that when Hb is broken down in the body, only part of it is excreted in the form of urobilinogen. This fact gives the right to assume that the breakdown products of the remaining part of the broken down Hb are used for building new Hb, that therefore a significant part—according to these data up to 50%—of the Hb newly produced in the body is built from materials obtained from the breakdown of Hb. The impression is gained that the organism just as carefully uses for building new Hb the already used pyrrole rings or complexes as is the case with Fe of the breaking down Hb.-What relation does the exchange of the pigment part of blood Hb have to the exchange of myoglobin, cytochrome and other hemochromogens like the respiratory enzyme of Warburg, about this we know nothing, just as we also do not know to what extent the exchange of all these substances is connected with each other. Pathology of pigment exchange of the Hb group. Pathological conditions of P. o. can be divided into disturbances of pigment formation and disturbances of its destruction. Pathological disturbances of the formation and breakdown of the pigment part of Hb should be distinguished from pathological disturbances of the formation and breakdown of Hb itself and of the red blood cells as a whole. In anemias, based on insufficient blood formation, we can speak of a disturbance of P. o. strictly speaking only when there are data in favor of a disturbance of the formation and breakdown of the pigment part of Hb. As for most anemias, caused by insufficient blood formation and excessive destruction of erythrocytes, we do not yet have such data and as the cause of insufficient blood formation we can rather assume a disturbance of the function of the nerve-humorally-hormonal apparatus regulating blood formation, various disturbances of other basic biological properties of those cells that produce Hb, disturbance of Hb formation in the sense of disturbance of the process of combining the pigment part with globin, lack of Fe etc. As the cause of excessive destruction of blood one can assume an intensification of hemolytic functions in the organism or a decrease in the resistance of erythrocytes to hemolytic influences. In most of these forms of anemia, as well as in opposite conditions—polycythemias, in which excessive blood formation or insufficient blood destruction can be assumed, there is therefore no primary disturbance of P. o. as the cause of anemia or polycythemias, but secondarily, quantitatively P. o. is of course disturbed also in these forms and even in anemias due to bleedings, i.e. the production of Hb by the organism, and consequently of the pigment part, as well as its disturbance increases or decreases and in connection with this the production of the bile pigment—bilirubin is also increased or decreased. The processes of blood formation and blood destruction are undoubtedly under the influence of a regulating apparatus in the organism and are closely connected with each other in the sense that intensified destruction of erythrocytes leads to intensified production of them, insufficient regeneration of erythrocytes or their loss by the organism due to bleedings leads to a decrease in hemolytic processes, and conversely, intensified production of erythrocytes causes their intensified destruction (in polycythemia) etc. What value the use of breakdown products of Hb for building new Hb has when there is a need for intensified regeneration of blood is best demonstrated by that great difference in the speed of blood regeneration that is observed between anemias due to intensified destruction of blood, on the one hand, and due to blood loss—on the other. The fact that with repeated blood loss blood regeneration occurs relatively very quickly leads to the assumption that building the pigment part from fresh material for the organism is considerably more difficult than from breakdown products of Hb. Blood regeneration after blood loss is apparently primarily hindered by the lack of Fe developing under these conditions, but the lack of organic component for forming the pigment part undoubtedly also plays a role. This follows from the classical experiments of Whipple and Robscheit-Robbins, who established that in dogs with severe anemias caused by repeated blood withdrawals, the speed of blood regeneration depends not only on the Fe content in the food but also on other peculiarities of its composition. Especially conducive to enhancing blood regeneration are liver, kidneys and gastric wall. According to these experimental data, green vegetables, containing chlorophyll close in chemical structure to the pigment parts of Hb, contribute to the formation of Hb to a significantly lesser extent than the animal food substances just mentioned. It is interesting that, conversely, the formation of chlorophyll, when at least cereals are grown, is greatly promoted by the addition of liver extracts to the nutrient solution (Suzman, Ungley). A completely special type of disturbance of blood formation occurs in malignant anemia. The data accumulated in the last 5 years in connection with the discovery of liver therapy give the right to attribute main importance in the pathogenesis of malignant anemia to the disturbance of erythropoiesis. As determined by Ehrlich long ago, erythropoiesis in this disease takes on a morphological character of embryonic erythropoiesis. Relatively recently Borst and Konigsdorffer determined that also with regard to the chemistry of the formation of the pigment part in malignant anemia there is an embryonic type of blood formation (see above). Another fact, also recently established, is the return of erythropoiesis in pernicious anemia to the normal type for extra-uterine life—both quantitatively and qualitatively—under the influence of the introduction into the organism of substances contained in particularly large amounts in the liver, kidneys and in the gastric wall of animals.

According to the latest data (Dakin, West and Howe), these organs contain a mixture of β-oxiglutamic acid and γ-oxiproline, i.e., amino acids which, as was stated above, can serve as material for the formation of the pigment particle. There are assumptions that pernicious anemia develops as a result of insufficient intake into the body of this substance necessary for the formation of Hb. This deficiency is apparently most often associated with a disturbance of digestion due to the gastric achylia present in these patients. With normal gastric digestion, this substance is obtained from ordinary food substances (mainly from muscle tissue), while with achylia it can only be obtained from the organs mentioned above. As a result of the deficiency of this substance necessary for that type of formation of the Hb pigment particle which takes place in extrauterine life, the construction of the pigment part of Hb in the body shifts to the embryonic type. Under the conditions of the adult organism, this type of blood formation, for reasons still unclear to us, remains completely insufficient. In the pathogenesis of pernicious anemia, according to this point of view, first place is occupied by improper erythropoiesis as a result of disturbance in the normal formation of the pigment part of Hb, while the intensified hemolysis is considered as a secondary phenomenon. Perhaps this point of view is too one-sided; the assumption that intensified hemolysis in pernicious anemia is the result only of decreased resistance of path. erythrocytes to normal hemolytic influences in the body is not substantiated by direct proof of this decreased stability of erythrocytes in pernicious anemia. Hemolysis in pernicious anemia in any case does not occur quite normally; in the blood in this disease, the presence of hematine (Schumm, Bingold) is regularly observed. In hemolytic jaundice or hemolytic anemia, particularly in its familial constitutional form, we have the right with considerable certainty to consider primary the path. blood formation with the production of erythrocytes differing morphologically in a special form—spherical and sharply decreased stability, due to which they quickly undergo normal hemolytic influences. The character of path. erythropoiesis here is completely different compared to pernicious anemia. It differs above all by its extraordinary intensity¹, thanks to which hemolysis despite the fact that it is increased much more strongly than in pernicious anemia, does not so easily, as in the latter, lead to the development of anemia. We know nothing about the disturbance in the construction of the pigment particle in constitutional hemolytic anemia. As a result of intensified hemolysis and the liver's inability to excrete all the bilirubin formed, this pigment accumulates throughout the body and jaundice develops. Incidentally, it should be noted here that besides hemolytic jaundice, other forms of diseases caused by a disturbance in the quantitative or qualitative formation of bilirubin have not yet been identified and clarified to a sufficient degree. According to the modern view, other forms of jaundice are caused by the retention of bilirubin in the body due to difficulty in its excretion or as a result of disturbance of the corresponding liver functions or due to mechanical obstacles in the bile ducts. From the point of view of pathology of P. o., attention is deserved by the anemia developing experimentally in dogs under the influence of constant loss of bile through a biliary fistula (Mc Master, Seyderhelm). After 2-4 weeks from the application of the fistula, anemia develops, and simultaneously the amount of bilirubin excreted with the bile decreases. Other data also indicate a decrease in erythropoiesis at this time. The thought arises that this anemia is the result of the body's loss of bile, containing in the form of bilirubin the material necessary for the construction of Hb. This thought, however, has not yet found confirmation in the corresponding experiments. The view still defended today that urobilinuria in liver diseases is a manifestation of path. formation of this pigment instead of bilirubin in this organ (Fischler) or from bilirubin in the blood and in all tissues (Brule) should still be considered unsubstantiated, since the formation in the body of urobilinogen in another place and from other material than in the intestine and possibly also in infected bile ducts, and only from bilirubin of bile, has not yet been proven. From the point of view of the classic theory of F. Müller (Fr. Müller), confirmed recently by precise experiments of Mc Master, Brown, Rous, urobilinogen is formed only in the intestine and only from bilirubin of bile. Urobilinuria is the result of disturbance of liver function, due to which urobilinogen, as already stated above, is not retained by it and penetrates into the great circle of circulation. It was stated above that in pernicious anemia hematine is determined in the blood. Many physiologists and pathologists (including Schumm) consider hematine an intermediate product in the formation of bilirubin from blood pigment. However, this formation of bilirubin from Hb through hematine should be considered unproven, since a whole series of facts speaks against this view: 1) it has not yet been possible to convert hematine into bilirubin artificially by chemical means; 2) it is not possible to prove the presence of hematine in the organs playing a role in pigment metabolism in such quantity as would be expected judging from the daily production of bilirubin; 3) hematine is determined in the blood only in certain path. conditions; besides in pernicious anemia hematine is observed in acute yellow atrophy of the liver, ectopic pregnancy, eclampsia of pregnancy, malaria, sepsis caused by the gas gangrene bacillus, in intoxications with Kali chloricum (Berthollet's salt), dinitrobenzene, acetic acid, toluylenediamine, phenylhydrazine (Schumm and others); 4) furthermore it should be noted that an increase in blood destruction in the body is not sufficient for the appearance of hematine, for example the latter is absent after blood transfusion when the transfused blood undergoes rapid destruction (Bingold, Nurenberger), in hemolysis in anaphylactic phenomena (Leblanc), in some cases of hemolytic jaundice (Bingold); experimentally it is not possible to cause hematine with toluylenediamine in all animals, e.g. in rabbits, despite significant hemolysis and sharp anemia after poisoning with toluylenediamine, hematine is not observed. All these facts speak in favor of the view that hematine is not a manifestation of intensification of the normal course of hemoglobin-pigment metabolism, but is a manifestation of the perversion of the latter occurring under the influence of certain toxic and toxo-infectious influences. Undoubtedly a path. product of the transformation of Hb is methemoglobin (see Hemoglobin). This transformation also occurs under the influence of poisonous substances, both exogenous, among which aniline derivatives stand in first place, e.g. antifebrin, phenacetin, nitrobenzene, nitrites, and endogenous, formed in the intestine in sharply expressed processes of decomposition in it. In the latter case a picture develops which consists of cyanosis and more or less severe intestinal phenomena (enterogenous pseudocyanosis) (Hijmans van den Bergh and Gutterink). In severe poisonings methemoglobin is excreted with the urine, and simultaneously severe kidney damage develops. The simultaneous presence of hematine and methemoglobin in the blood and urine is often observed. To the diseases which secondarily disturb the P. o. of hemoglobin belong also all those path. conditions when erythrocytes circulating in the blood undergo disintegration, when path. hemolysis occurs and hemoglobinemia results (see Hemoglobinuria, hemoglobinemia). Methemoglobinuria resulting from methemoglobinemia—the transformation of Hb into methemoglobin in the circulating blood—should be distinguished from methemoglobinuria resulting from the secondary transformation of Hb into methemoglobin, as is often observed in various hemolytic processes without the formation of methemoglobin in the blood, i.e. without methemoglobinemia. The Hb liberated during hemolysis is partially excreted by the kidneys—hemoglobinuria, partially captured by the endothelium of the liver and spleen, where it undergoes further changes. Of special theoretical interest is the so-called paralytic hemoglobinuria or rather myoglobinuria. This disease is well known as a disease of horses and more rarely of horned cattle, but occasionally occurs in humans. It consists of myoglobinemia and myoglobinuria due to the exit of myoglobin from the muscle in a severe paralytic state of the musculature, which on section is found to be almost discolored and under the microscope reveals marked degenerative changes. The most pronounced disease associated with disturbance of P. o. is porphyria. Physiology and pathology of pigments not belonging to the Hb group. Besides porphyria a pronounced disease of P. o. is hemochromatosis (see).

It has been established that in ochronosis two types of pigment are deposited in the body: one containing iron of the hemosiderin type, and the other not containing iron (Bork) or containing it in a form that can only be detected after treatment with ammonium sulfide (Hueck). The first pigment is deposited mainly in the liver and spleen, but also in many other organs; the second is deposited mainly in smooth muscle and in the brain. The deposition of the first pigment is considered to be the result of a reduced ability of the corresponding cells (primarily the reticulo-endothelium) to absorb or rather to process iron into a form that can be used for the construction of the pigment complex. The origin and nature of the second pigment are unclear. According to Hueck, it belongs to the lipoid substances—he calls it lipofuscin; according to Bork, it belongs to the melanin group—hence of protein origin. Bork considers it identical with the normal pigment that accumulates as a result of wear processes in the corresponding organs, mainly in the muscles. He attributes its appearance and deposition in excessive amounts to a certain toxic influence underlying the entire disease, which also leads to a disturbance of iron metabolism. Another pigment or group of pigments, the disturbance of whose formation and destruction is of practical interest, is melanin (see). Close to the melanin group is the pigment that forms in alkaptonuria (see) from the homogentizic acid accumulating in the body and causes the so-called ochronosis (see). Homogentizic acid is formed as an intermediate product in the breakdown of tyrosine and phenylalanine in the body; in alkaptonuria, the body loses the ability to further transform homogentizic acid. As mentioned above, melanin is also a derivative of these amino acids according to Thannhauser, but in tissues of ectodermal origin (skin and adrenal glands), the breakdown process proceeds differently—specifically through protocatechuic acid, which in turn is transformed into pigment. When homogentizic acid accumulates in tissues, in some (not all) alkaptonurics it is also transformed into a blue-black pigment. The accumulation of this pigment occurs in tissues poor in blood vessels, mainly in cartilage and generally in joint tissues, accompanied by severe chronic osteoarthropathy. In living patients, a corresponding discoloration of the sclerae, nasal wings, and ear shells can sometimes be noticed (see Ochronosis). A pigmentation completely analogous in character and localization is observed in chronic poisoning with carbolic acid, when a corresponding pigment is formed in the body from phenols also through oxidative processes. The localization of the pigmentation is due to a special arthropathy common to phenols, homogentizic acid, and salicylic acid. The disturbance of metabolism of the third group of pigments—those accompanying fats, the so-called lipochromes (see)—has the least practical significance. It is not yet known to what extent these pigments are only of exogenous origin, i.e., are introduced with food (such as the lipochrome of egg yolk, butter, or the carotene of carrots), and to what extent the body is capable of synthesizing them. Lipochromes are always present in the plasma or blood serum in greater or lesser amounts. After extirpation of the liver in dogs, a yellow pigment appears in the blood and tissues, which, according to Thannhauser, Enderlen, and Jenke, also belongs to the lipochromes. They explain its accumulation in the body after removal of the liver by the fact that the latter has the ability to capture these pigments from the blood, subject them to breakdown, and excrete them with bile. Noticeable pigmentation by lipochromes of the palms and soles is sometimes observed in diabetics on a diet rich in plant food (xanthosis diabetica). The intensely yellow color of the so-called xanthelasma is also caused by lipochromes.