CHEMICAL CLASSIFICATION OF KERATINS* By RICHARD J. BLOCK Department of Biochemistry, New York Medical College, Flower and Fifth .dyehue Hospitals, New York, 29, N.Y. IT •s BECOMES6 increasingly apparent that the total proteins of an homologous series of tissues have a remarkably constant amino acid composition. Table 1 shows the amino acid patterns of washed and fat-extracted muscle proteins from rived, have the same amino acid pattern. An analogous situation appears to hold in the case of milk from the few species that have been investigated (Table 2). Of the twelve amino acids studied only cystine appears to show a significant TABLE 1--SOME AMII•O Acres xs Musc•.• PROTEII•S , Gm. per 16 Gm. of N.-- , Beef Chicken Frog Fish Shrimp Arginine 6.9 6.9 6.6 6.4 6.6 Histidine 2.3 2.3 2.1 2.3 1.8 Lysine 8.1 8.4 8.0 9.0 8.3 Tyrosine 4.3 4.2 4.7 4.4 4.7 Tryptophan 1.4 1.3 1.4 1.4 1.2 Phenylalani ne 4.9 3.9 4.7 4.5 4.8 'Cystine 0.9 0.8 0.9 0.7 1.0 Methionine 3.1 3.4 3.2 3.8 3.4 Serine 5.4 4.7 6.3 4.0 4.0 Threonine 4.6 4.7 4.9 4.2 4.0 Leucine 9.2 ...... 7.3 ... Isoleucine 4.8 ...... 5.0 ... Valine 6.0 ...... 5.6 ... various classes of vertebrates--- mammals, birds, amphibia, fish, and crustacea. It will be seen that within the experimental error of the method there is no difference in amino acid pattern over practically the entire range of animals. Thus the total proteins of muscle, from whatever animal .they may be de- * Presented at the May 19, 1950 Meeting in New York City. deviation. Table 3 shows that even the amino acid patterns of entire animals and birds are, likewise, very similar. The collagens and elastins, obtained from mammals, marsup- ials, birds, amphibia, and fish have been analyzed for eighteen amino acids under strictly comparable conditions. Of all the collagens and elastins studied, only fish skin collagen showed a significant devia- 235

236 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TABLE 2--SOME AMINO ACIDS IN MILK PROTEINS •Gm. per 16 Gm. of N-- Human Cow Ewe Arginine 3.7 3.4 3.2 Histidine 2.7 2.6 2.7 Lysine 7.2 8.3 8,4 Tyrosine 5,4 6.4 Tryptophan 1.5 1.4 Phenylalanine 5.3 4.7 4.7 Cystine 1.6 0.7 Methionine 2.2 2.7 2.5 Threonine 4.5 4.5 4.3 Leucine 9.8 9.0 9.6 Isoleucine 7.5 5.7 5.5 Valine 8.8 7.0 7.4 TABLE 3--SOME AMINO ACIDS IN WHOLE ANIMALS •Gm. per 16 Gm. of N--- Rat Chicken Arginine 5.9 5.8 Histidine 1.8 1.5 Lvsine 6.1 6.3 Phenylalanine 3.4 4.2 Methionine 2.0 1.6 Threonine 3.7 3.7 Leucine 6.4 7. :2 Isoleucine 3.8 4.2 Valine 4.5 5.4 Glutamic Acid 13.9 11.6 Aspattic Acid 8.1 8.2 Glycine 10,0 11.0 tion in three of the eighteen amino acids (methionine, serine, and threonine) while turtle collagen de- viated from the majority only in threonine. Other tissue proteins such as liver, stroma, hemoglobins, etc., show remarkably similar am- ino acid patterns and if any marked differences are encountered they are usually confined to the sulfur amino acids and possibly to the hydroxyamino acids. A group of tissue proteins of ecto- dermal origin such as hair, nails, horn, feathers, etc., have been recognized for the past seventy-five years as having certain features in common. These proteins are re- sistant to digestion by pepsin and by trypsin and are insoluble in dilute acids, alkalies, water, and or- ganic solvents. They are called keratins from the Greek word "Keras," horn. Studies initiated about twenty years ago showed that keratins could be divided into two groups which had approximately the same physical properties but which differed in the molecular ratios of their basic amino acids, arginine, TABLE 4•AMINO ACIDS IN HAIR KEKATINS . Gm. per 16 Gm. of N- Human Hair Chimpanzee Hair Swine Hair Lamb's Wool Arginine 9.6 8.1 10.5 10.1 Histidine 0.9 0.6 1.1 0.9 Lysine 2.6 2.0 3.7 2.8 Tyrosine 3.1 3.3 3.4 5.5 Tryptophan 0.7 1.4 0 7 Phenylalanine 2.7 •} 6 4.0 Cystine 16.0 1• 15 13.9 13.6 Methionine 1 0 .. 0.5 0.7 Setinc 7.6 .. 9.0 Threonine 7.2 ., •' 1 6.5 Leucine .... 8.0 7.8 Isoleucine .... 4.5 4.2 Valine .. 5.7 5.1 Glutamic Acid lz[ 18 .. 17.2 15.4 Aspattic Acid 8.0 .. 7.7 7.1 Proline .... 9.3 8.0