Effect of substrate concentration on inhibition of prostaglandin synthetase of bull seminal vesicles by anti-inflammatory drugs and fenamic acid analogs

Abstract

Although microsomes of bull seminal vesicle synthesize prostaglandins F 2α, E 2, and D 2 from arachidonic acid under suitable assay conditions, prostaglandin E 2 is the only significant product at either low concentration of arachidonic acid or high concentration of microsomes. Studies of inhibition of prostaglandin synthesis in vitro by anti-inflammatory drugs at both high (1 mM) and low (1 μM) concentrations of arachidonic acid, suggest three distinct mechanisms of inhibition. Benzydamine and flazalone are non-competitive or weakly competitive with arachidonic acid and, at high concentrations of arachidonic acid, they augment synthesis of prostaglandin E 2 while inhibiting production of prostaglandins F 2α and D 2. Niflumic acid and the arylacetic acids naproxen and ibuprofen are competitive inhibitors, inhibiting all products equally, but with 100–500-fold greater potency at the low substrate concentration. The fenamic acids, indomethacin, aspirin, and phenylbutazone also inhibit equally all prostaglandin products, but are only 20–50 times more potent at the low substrate concentration. Studies with analogs of the fenamic acids indicate that the diphenylamine portion of their structure is essential for inhibition of prostaglandin synthesis, whereas the o-carboxyl and m-alkyl substituents greatly enhance inhibitory potency.