Umezawa Hamao

Inhibition of dopamine β-hydroxylase by fusaric acid (5-butylpicolinic acid) in vitro and in vivo

Abstract Fusaric acid (5-butylpicolinic acid) was found to be a potent inhibitor of dopamine β-hydroxylase in vitro and in vivo . Fusaric acid inhibited the enzyme by 50 per cent at a concentration of 3 × 10 −8 M. Kinetic studies with purified dopamine β-hydroxylase showed that the inhibition by fusaric acid was of the uncompetitive type with the substrate and of the mixed type with a cofactor, ascorbic acid. Fusaric acid lowered endogenous levels of norepinephrine and epinephrine in brain, heart, spleen and adrenal glands. Maximum depletion of norepinephrine and epinephrine was observed between 3 and 6 hr after the administration of fusaric acid. Since dopamine β-hydroxylase activity in adrenal medulla was found to be inhibited in vivo after the administration of fusaric acid, the decrease in the catecholamine levels is attributed to the inhibition of norepinephrine synthesis at the dopamine β-hydroxylase stage.

Inhibition of brain prostaglandin d synthetase and prostaglandin D2 dehydrogenase by some saturated and unsaturated fatty acids

The activities of rat brain prostaglandin D synthetase and swine brain prostaglandin D2 dehydrogenase were inhibited by some saturated and unsaturated fatty acids. Myristic acid was most potent among saturated straight-chain fatty acids so far tested. The IC50 values of this acid were 80 microM for prostaglandin D synthetase and 7 microM for prostaglandin D2 dehydrogenase, respectively. Little inhibition was found with methyl myristate and myristyl alcohol. The IC50 values of these derivatives were more than 200 microM for both enzymes, suggesting that the free carboxyl group was essential for the inhibition. The effects of cis double bond structure of fatty acids on the inhibition potency were examined by the use of the carbon 18 and 20 fatty acids. The inhibition potencies for both enzymes increased with the number of cis double bonds; the IC50 values of stearic, oleic, linoleic and linolenic acid were, respectively, more than 200, 60, 30 and 30 microM for prostaglandin D synthetase, and 20, 10, 8.5 and 7 microM for prostaglandin D2 dehydrogenase. Arachidonic acid also inhibited the activities of both enzymes with respective IC50 values of 40 microM for prostaglandin D synthetase and 3.9 microM for prostaglandin D2 dehydrogenase, while arachidic acid showed little inhibition. The kinetic studies with myristic acid and arachidonic acid demonstrated that the inhibition by these fatty acids was competitive and reversible for both enzymes. Myristic acid and other fatty acids also inhibited the activities of several enzymes in prostaglandin metabolism, although to a lesser extent. The IC50 values of myristic acid for prostaglandin E isomerase, thromboxane synthetase and NAD-linked prostaglandin dehydrogenase (type I) were 200, 700 and 100 microM, respectively. However, this fatty acid showed little inhibition on fatty acid cyclooxygenase (20% at 800 microM), glutathione-requiring prostaglandin D synthetase from rat spleen (20% at 800 microM), and NADP-linked prostaglandin dehydrogenase (type II) (no inhibition at 200 microM).