ISSN 1017-2548(Print)
ISSN 2234-8530(Online)
Volume 19, Number 4
JKCSEZ 19(4)
August 20, 1975 

A spectrophotometric assay technique is descriead for the measurement of free SH-groups in the enzyme reaction mixture. The method utilizes a new substrate, S-hippuryl-thioglycolyl-glycine(S-Hip-thioglycol-Gly) which is the basis for a convenient assay of angiotensin-converting enzyme and other dipeptidyl carboxypeptidases. This substrate contains an appropriately located thioester linkage that is hydrolyzed by the converting enzyme and other dipeptidyl carboxypeptidases. One of the products, thioglycolyl glycine, is readily measured by reaction with Ellman's reagent, 5,5'-dithio-bis-(2-nitrobenzoic acid), DTNB, to produce 5-thio-2-nitrobenzoic acid which has a strong absorption band at 410 nm. The method is sensitive (εM = 1.36×104 at 412 nm) and can be applied as a continuous recording with DTNB present in the enzymatic reaction mixture.