In vitro studies on the metabolic pathway of SQ 14225 (captopril) and mechanism of mixed disulfide formation.

The metabolic pathway of SQ 14225 (Captopril) and the mechanism of the mixed disulfide formation with endogenous sulfhydryl compounds were studied in in vitro cell free systems. In the rat liver 9000 x g supernatants, SQ 14225-14C was metabolized to one major metabolite, glutathione-SQ 14225 mixed disulfide (GSSQ), and two minor metabolites including SQ 14551, a symmetrical disulfide of SQ 14225. The formation of GSSQ was markedly accelerated by the addition of oxidized glutathione (GSSG), but not affected by the addition of reduced glutathione (GSH), indicating that GSSQ was formed by the thiol-disulfide interchange between SQ 14225 and GSSG. Although the thiol-disulfide interchange was also observed between SQ 14225 and L-cystine, L-homocystine and SQ 14551 as well as GSSG, only the formation of GSSQ was a rapid reaction and markedly decreased by heat treatment of the liver 9000 x g supernatants. These findings demonstrate that the formation of GSSQ is catalyzed by a GSSG specific enzyme which is supposed to be thioltransferase (Glutathione: Disulfide Oxidoreductase). Although GSSQ was stable in the rat liver 9000 x g supernatants, it was rapidly hydrolyzed to cysteine-SQ 14225 mixed disulfide (CySSQ) in the rat kidney 9000 x g supernatants. A specific inhibitor of gamma-glutamyltranspeptidase, anthglutin, inhibited the hydrolysis. GSSQ-14C administered to a beagle dog was excreted into the urine in the form of CySSQ. Thus, it was speculated that CySSQ excreted into the urine as a major metabolite of SQ 14225 was derived from GSSQ formed in the liver followed by hydrolysis in the kidney.