Abstract

Peptidylglycine-α-hydroxylating monooxygenase (PHM; EC 1.14.17.3) catalyzes the first and rate-limiting reaction in the two-step process that α-amidates neural and endocrine peptides. The substrate analog 4-phenyl-3-butenoic acid (PBA) was shown in vitro to selectively inhibit PHM without affecting the activity of peptidyl-α-hydroxyglycine α-amidating lyase, the enzyme that mediates the second reaction in α-amidation. Inhibition of PHM activity by PBA lowered the V_max of the enzyme without altering its K_m. Administration of PBA in vivo profoundly inhibited serum PHM activity in a dose- and time-related fashion. Maximal reductions to less than 5% of control levels were observed 3 h after a single administration (500 mg/kg). Inhibition of serum PHM activity by PBA was short-lived, being fully reversed by 24 h postinjection. PHM activity in cardiac atrium, hypothalamus, and anterior and neurointermediate lobes of the pituitary were also decreased by PBA treatment but to a lesser extent than with serum. Inhibition of PHM activity by PBA was not cumulative over time when assessed 24 h after the last of 10 daily injections (500 mg/kg). The role of protein synthesis in maintaining PHM activity in blood was demonstrated by treatment with cycloheximide, which reduced serum PHM activity and retarded the recovery of PHM activity after PBA administration. It is concluded that the metabolism and/or clearance of PBA is rapid and that de novo protein synthesis has an important role in mediating the rapid restoration of PHM activity after PBA administration.