Clxxxix. Carbohydrates in Proteins I. the Carbohydrate Component of Crystalline Egg Albumin

THE question of the occurrence of carbohydrate groups as essential parts of the structure of proteins (apart from mucins and mucoids) has been the subject of many investigations, none of which however has yielded an unequivocal result. Before the peptide theory of proteins was generally accepted, numerous papers were published conceming the carbohydrate content ofiproteins. Pavy t1893] hydrolysed coagulated egg-white first with alkali and then with dilute acid and obtained glucosazone from the hydrolysate. He assumed that all proteins contain fairly large amounts of carbohydrate and that the preformed sugar in the protein molecule is responsible for the conversion of protein into carbohydrate which occurs in the animal body. These contentions were the subject of a heated controversy and, although Pavy's physiological hypotheses were somewhat bizarre, his chemical observations were correct. Eichholz [1898] confirmed Pavy's findings by showing that egg albumin, from which mucin and mucoid had been carefully removed, gave on hydrolysis a substance which reduced Fehling's solution and from which an osazone of M.P. 2080 could be obtained. Later, when crystalline egg albumin had been prepared, Hofmeister [1898] and Langstein [1902] obtained an osazone from the hydrolysate of the crystalline protein and Seemann [1900] isolated glucosamine hydrochloride from a thrice crystallized. sample. The amounts isolated, however, were very small and it was generally assumed that ordinary proteins are composed of aminoacids only and that the presence of sugars in hydrolysates is due to impurities such as mucoids (see Levene [1923] and Plimmer [1917]). Much later, the question was taken up again by Frankel & Jeilinek [1927] who hydrolysed apparently non-crystalline egg albumin with baryta, removed amino-acids with neutral lead acetate and precipitated the polysaccharide with ammoniacal lead acetate. They repeated this separation several times and finally obtained, by precipitation with alcohol, an optically inactive polysaccharide. This substance on acid hydrolysis gave mannose and glucosamine. Levene & Mori [1929], following the work of Frankel & Jellinek, introduced as an additional step in the purification of the polysaccharide the use of mercuric sulphate for the removal of nitrogenous impurities. These authors, however, obtained only small amounts of polysaccharide from repeatedly recrystallized egg albumin and concluded therefore that the carbohydrate is not derived from the egg albumin itself but from ovomucoid. Rimington [1929; 1931] applied the same method of hydrolysis and isolation to mixed serum proteins and also obtained mannose and glucosamine hydrochloride as ultimate hydrolysis products.