122 



PROTEINS 



denotes the remainder of the formula, e.g., CH3 for alanine, etc. Formerly 

 the small letters d and I were used to denote the two forms, but since 

 these letters are also used to indicate optical rotation, the American and 

 British chemical societies have adopted small capital letters to show 

 configuration. The reference compounds, d and l serine, correspond to 

 those used in sugar chemistry, viz., d and l glyceraldehyde. Optical 

 rotation is indicated by the words dextro and leva, or by plus ( + ) and 

 minus ( — ) signs. 



Most of the amino acids are of the l type, but small amounts of the d 

 acids have been reported in several common proteins. Cancerous tissue 

 has been reported to be much higher in D-glutamic acid than normal 

 tissue, but the weight of evidence seems to be against this conclusion. 

 Several antibiotics, e.g., gramicidin, tyrocidine and actinomycin, contain 

 large amounts of D-phenylalanine, D-leucine, and D-valine. Other anti- 

 biotics, e.g., penicillin and polymyxin, contain the previously unknown 

 amino acids, D-penicillamine and D-a,y-diaminobutyric acid, respectively. 

 The presence of unusual structures of amino acids, sugars, etc., seems to 

 be a general characteristic of antibiotics. 



Some of the amino acids listed have not yet been found in proteins. 

 For a long time a number of these were assumed not to occur in pro- 

 teins, but recently several have been found in polypeptides, e.g., or- 

 nithine in gramicidin and lanthionine in subtilin (an antibiotic). Homo- 

 cysteine, as yet unreported in any protein, is an intermediary product in 

 the conversion of methionine to cysteine. Cystathionine is also an inter- 

 mediate in this conversion. The steps are 



+ serine 



methionine > homocysteine > cystathionine 



> cysteine and homoserine (in neurospora) or 



a-ketobutyric acid (in animals) 



Amino acid composition of proteins 



To determine the amino acid content of a protein, it must first be 

 hydrolyzed. This is usually done with about twenty per cent hydrochloric 

 acid at 100°C. for 10 to 20 hours. The acid is then removed, and the 

 amount of each amino acid is determined quantitatively. This analysis 

 is one of the most difficult tasks in analytical chemistry, having defied 

 the efforts of some of the world's ablest chemists for the past seventy- 

 five years. Only within the last decade have methods been perfected to 

 such an extent that all the nitrogen or sulfur in a protein can be accounted 

 for in the amino acid figures. The most promising methods at present 

 seem to be microbiological and chromatographic. Bacteria are most 

 widely used in microbiological assays, and the procedures are the same 

 as those used in analyzing for vitamins (p. 234) . Chromatographic pro- 



