Inhibition of hepatic drug-metabolizing enzymes by arachidonic acid.

1. The effects of arachidonic acid on hepatic drug-metabolizing enzymes was investigated in male ICR-Swiss mice. 2. A single administration of arachidonic acid, 100 mg/kg i.p., doubled the hexobarbital sleeping time. Arachidonic acid in vitro gave a type I binding spectrum with hepatic microsomes; it inhibited the metabolism of hexobarbital and of ethylmorphine, two type I binding drugs, but not that of aniline, a type II binding drug; the inhibition of hexobarbital metabolism by arachidonic acid was competitive. 3. Repeated administration of arachidonic acid up to a total dose of 1000 mg/kg i.p., either in the course of 5 hours, or in the course of 5 days, decreased microsomal cytochrome P-450 levels and NADPH-cytochrome c reductase activity. 4. It is concluded that the administration of arachidonic acid may impair drug metabolism in two ways, mainly, by competitively inhibiting the activity of drug-metabolizing enzymes, and secondarily, by decreasing the hepatic concentration of these enzymes.