Nucleins

NUCLEINS, see Nucleoproteids, NUCLEOALBUMINS (in English and American terminology—phosphoproteins) belong to the group of phosphorus-containing complex proteins (proteids); this group of proteins should not be confused with nucleoproteids (see), from which N. differ by the absence of nucleic acid. Unlike phosphorus-containing glycoproteins, N. do not give reducing substances when boiled with mineral acids. The phosphorus of N. is easily cleaved off in the form of H₃PO₄ when acted upon by 1% NaOH, is partially cleaved off by trypsin, even less by papain, and is not cleaved off at all by acids. When acted upon by pepsin and hydrochloric acid, phosphoproteins first dissolve, partially turning into albumoses, then the part containing phosphorus is cleaved off; this part first precipitates, then dissolves and is called paranuclein or pseudonuclein, sometimes paranucleic acid. Paranuclein is not a pure substance but a mixture; it contains more phosphorus than the original substance, has distinctly acidic properties, dissolves in alkalis, and is precipitated by acids. Unlike nucleins, it dissolves in Ba(OH)₂, easily turning into acidalbumin and albumoses, liberating H₃PO₄. Paranuclein precipitates protein itself and is precipitated by salts of heavy metals and some alkaloid reagents. When acted upon by trypsin, paranuclein dissolves; it is absorbed in the intestines. Nucleoalbumins have an acid reaction to litmus paper, are almost insoluble in water, dissolve in alkalis and are precipitated from these solutions by acids. Some of the N. (e.g. casein) in their properties approach alkaline albuminates, others (e.g. vitellin) approach globulins. Solutions of N. salts do not coagulate when boiled. As amphoteric electrolytes, N. form salts with acids. The amino acid composition of N. varies among different representatives. N. include casein, vitellin (see), ichthulin (phosphoglycoprotein of fish roe), phosphoproteins of cellular protoplasm and mucin-like phosphoproteins. N. are proteins that serve as nutrition for developing animal and plant organisms.