Affinity

AFFINITY (from Latin affinitas—affinity, close connection), a concept used to explain the selective relationship of cell protoplasm to certain poisons or, in general, to various chemical substances. For example, CaCl₂, which is poisonous to muscles and nerves, is harmless to ciliated epithelium, spermatozoa, and many plant cells (Overton). Curarine, which paralyzes the endings of motor nerves in mammals and even in frogs, is ineffective for all invertebrates examined to date (Straub). Frog tissues are insensitive to cantharidin. One of the most potent poisons—tetanus toxin—is indifferent to the turtle. It is also customary to explain the fact of the selective action of poisons on specific organs in higher animals by affinity. The doctrine of affinity in this sense is based, mainly, on data regarding the specific physiological effect produced by poisons, but there are other supporting points for it, although they are scarcer. Undoubtedly, chemical affinity lies at the basis of the action of carbon monoxide, which enters into a compound with the hemoglobin of red blood cells. In other cases, a preferential accumulation of poison in a specific organ is noted: for example, in the spinal cord of frogs poisoned with strychnine, this poison was found in much larger quantities than in all other organs. However, the fact of such a concentration of poison in itself is by no means sufficient to conclude the presence of chemical affinity between the latter and the organs, since the accumulation can be caused by other factors, for example, physical conditions of solubility or adsorption capacity, as is evident from the classical studies of Ehrlich on the vital distribution of dyes in the organism and the works of H. Meyer and Overton on narcosis. When feeding mice with derivatives of paraphenylenediamine, brown staining is found only around the center of the diaphragm, but not in its peripheral parts, and then on the eye, larynx, and tongue. The same peculiarity in distribution is noted with vital staining by methylene blue: the endings of motor nerves of voluntary muscles are not stained in such cases, with the exception, however, of those same muscle groups of the eye, larynx, and diaphragm. Ehrlich interpreted this phenomenon as a difference in the saturation of various parts of the body with oxygen depending on the degree of blood supply. So-called neurotropic dyes (methylene blue, Bismarck brown, Neutral red, etc.) stain the brain vitally, firstly, by virtue of the fact that they are basic and therefore are not bound by the blood, which has an alkaline reaction, and secondly, because of their "lipotropicity"—solubility in fats; the brain is rich in lipoids. In the action of narcotics, according to Meyer and Overton, the so-called coefficient of solubility of a substance in fat and water is of great importance: the higher the former, the lower the concentrations in which the substance acts on the brain. According to Ehrlich, the relationships between toxin and antitoxin, as well as between toxin and the tissues of an animal sensitive to it, are determined by purely chemical affinity: thus, if a solution of tetanus poison is mixed with an emulsion of fresh rabbit brain and centrifuged after standing for a short time, the poison will be found fixed to the tissue, and both the brain mash and the liquid above it will be non-poisonous (experiment of Wassermann and Takaki). Other scientists interpret these facts differently: the first is explained by adsorption, and the second by the neutralization of the poison by lipoids. The phenomena of affinity were later widely used by Ehrlich in his research on chemotherapy (see), which proved the possibility of increasing its affinity either to a parasite or to one or another organ, or depriving it of activity altogether, by introducing one or another chemical group into a preparation.