A titrimetric method for the determination of creatine phosphokinase

residual colour is almost wholly accounted for by its tyrosine and tryptophan content. Thus the major part of the colour yielded by proteins must arise from chromnogenic amino acid sequences. It had seemed possible, at the outset of this investigation, that the large colour increments of proteins might be ascribed to a rather restricted set of sequences. Had that proved to be the case, the Lowry determination might conceivably have been used specifically to detect such sequences in poly- peptides. However, the results with chain B of insulin, as well as those with simple dipeptides, point to the existence of a considerable number of different chromogenic sequences in proteins. SUMMARY 1. In the Lowry protein determination the phenol reagent is reduced to yield a blue colour. The colour yield of a completely hydrolysed protein is due to its content of tyrosine, tryptophan and cysteine, the only amino acids that react signifi- cantly.