Peroxidase catalysed formation of prostaglandins from arachidonic acid

Abstract

Horseradish peroxidase and bovine lactoperoxidase (EC 1.11.1.7), when incubated aerobically with arachidonate, gave rise to the formation of substances identified by bioassay as prostaglandin F 2α (PGF 2α)- and prostaglandin E 2 (PGE 2)-like compounds. Boiling of enzymes, which suppressed their capacity to peroxidise guaiacol, also destroyed their capacity to convert arachidonate into PG-like compounds. The rates of formation of PG-like compounds rapidly declined with time, approaching zero after 10 and 20 min for PGF 2α- and PGE 2-like compounds, respectively. Addition of more enzyme further promoted the reaction. Horseradish and lacto-peroxidases showed optimum pH values of 9.0 and 10.0, respectively. Both enzymes exhibited apparent K m values of about 5 × 10 −5 M for arachidonate. Some reducing agents such as ascorbic acid, NADH and adrenaline dose-dependently inhibited this reaction. The haem poison, phenylhydrazine, also inhibited, with an ic 50 of 1 × 10 −7 M. Indomethacin inhibited only the formation of PGE 2-like compounds with an ic 50 of about 3 × 10 −6M. As compared to a standard commercial preparation of horseradish peroxidase, the purified horseradish basic and acidic isoenzymes exhibited a higher activity, towards arachidonate whereas other haemoproteins, possessing peroxidase activity, were less active. TLC and GC-MS analyses performed on the reaction products led to the identification of PGF 2α, PGE 2 and PG6K 1α and other unidentified arachidonate derivatives. At 25°, pH 9.5, horseradish peroxidase, acting on saturating concentration of arachidonate, catalysed the formation of 60 μmol/min/mmole enzyme of PGE 2 + PGF 2α. This appears to be the first report of the synthesis of prostaglandins catalysed by peroxidases.