Thiorphan, tiopronin, and related analogs as substrates and inhibitors of peptidylglycine α-amidating monooxygenase (PAM)

Peptidyglycine α-amidating monooxygenase is a copper- and zinc-dependent, bifunctional enzyme that catalyzes the cleavage of glycine-extended peptides or N-acylglycines to the corresponding amides and glyoxylate. This reaction is a key step in the biosynthesis of bioactive α-amidated peptides and, perhaps, the primary fatty acids amides also. Two clinically useful N-acylglycines are thiorphan and tiopronin, each with a thiol moiety attached to the acyl group. We report here that thiorphan and tiopronin are substrates for PAM, exhibiting relatively low K M,app and V MAX,app values. The low V MAX,app values result, most likely, from a decrease in active PAM · 2Cu(II) as the enzyme competes ineffectively with thiorphan and tiopronin for free copper.