258 H. FRAENKEL-CONRAT AND K. NARITA [14 



enzyme specificity, but was occasionally ascertained by hydrazinolysis^^-^^ 

 or carboxypeptidase. Sequences have been determined in only a limited 

 number of peptides, ranging from dipeptides to hexapeptides.^*'-^^ One hexa- 

 peptide isolated from chymotryptic digests as the DNP derivative appeared 

 of particular interest because it proved to represent the C-terminal peptide 

 of the protein. ^^ The same hexapeptide was obtained from the masked and 

 YA strains, but one of slightly different composition was obtained from 

 the H.R. strain. 



(4) The identified fragments are devoid of key amino acids and too small 

 to permit even a beginning of the final stage of the work, the fitting together 

 in proper sequence. The disturbing possibility must also be faced that all 

 peptide chains need not necessarily be identical. An appreciable hetero- 

 geneity within that population would render our aim, as now approached, 

 unrealizable. At present, one can only be sure that all or almost all peptide 

 chains have the same C-terminal sequence. It is also definitely established 

 and agreed*"-*^ that none or almost none of the chains has a free A^-terminal 

 group. Several preliminary notes by Braunitzer and Schramm discussing the 

 nature of the 'hidden' A^-terminal group and describing a crystalline A^- 

 terminal trypsin fragment of the protein have not been confirmed. Some 

 of the claims and ideas were most attractive, but they had to be discarded 

 for lack of factual support. 



The absence of an A^-terminal group raises the possibility that digestion 

 of the protein by enzymes other than trypsin might produce peptides which 

 are devoid of any basic group. A search for such peptides was therefore 

 initiated by one of us (K.N.)*^ by passing various digests through the acid 

 form of Dowex-50-X2 columns. The same main component of this acid 

 peptide fraction was isolated from either chymotryptic or peptic digests, and 



240 



320 

 m|A. 



400 



Fig. 4. Ultraviolet absorption spectrum of DNP-derivative of acidic peptide, and of 

 0-DNP-tyrosine. 



