Action of Cathepsin C on Dipeptide Esters *

In earlier studies"2 on the purification and properties of beef spleen cathepsin C, it was found that at pH 5 this proteolytic enzyme catalyzes the hydrolysis, not only of the CO-NH bonds of sensitive substrates, but also of the ester linkage of a-L-glutamyl-L-tyrosine ethyl ester. In view of the subsequent finding4 that cathepsin C exhibits pronounced specificity toward dipeptide derivatives (e.g., glycyl-L-phenylalaninamide [GPA]), but does not act on related amino acid amides (e.g., L-phenylalaninamide) or tripeptide amides (e.g., glycylglycyl-L-phenylalaninamide), it seemed of interest to examine more closely the action of the enzyme upon several dipeptide esters both at pH 5 and at pH 7.5. Previous work9 had shown that at pH 5 the predominant reaction is one of hydrolysis at the sensitive amide bond of substrates such as GPA; at pH values near 7.5, the major reaction is one of transamidation' leading to the synthesis of long-chain polypeptides. Thus, at pH 7.5, GPA is converted by the enzyme to a polymeric product that is, on the average, an octapeptide composed of alternating glycyl and L-phenylalanyl residues. In Table 1 are given the results of experiments in which several dipeptide esters were subjected to the action of two preparations of cathepsin C, kindly provided by Dr. Peter S. Cammarata and Dr. Gabriel L. de la Haba of this laboratory.t These two enzyme preparations differed in their specific activity toward GPA; preparation I contained 32.8 cathepsin units per mg. N, whereas preparation II contained 9.6 cathepsin units per mg. N.t It will be seen from Table 1 that when equal amounts of enzyme, as measured by the action on GPA, are used, the two preparations are equally