Enzymic Synthesis of Juvenile Hormone in Locust Corpora Allata: Evidence for a Microsomal Cytochrome P‐450 Linked Methyl Farnesoate Epoxidase

Abstract

Homogenates of corpora allata from adult Locusta migratoria in phosphate-buffered EDTA have been analysed by sucrose-density-gradient centrifugation. Succinate-cytochrome c reductase activity (mitochondrial) bands between d²⁰₄ 1.13–1.15, whereas NADPH-cytochrome c reductase and NADPH–dependent methyl farnesoate 10, 11-epoxidase activities band identically between d²⁰₄ 1.06–1.12. We conclude that the methyl farnesoate epoxidase is exclusively microsomal. Farnesoic acid O-methyltransferase is an exclusively soluble enzyme which stoichiometrically transfers the S-methyl group from S-adenosylmethionine to farnesoic acid. No carboxyl esterase activity was found. Isolated microsomes were used to obtain an apparent K_m= 7.7 × 10⁻⁶ M for the epoxidase, although substrate solubility limits the rate to 0.5 V As expected, the product (juvenile hormone III) is chiral (10 R). The epoxidase is inhibited by excess NADP⁺ and oxidised cytochrome c, but neither inhibited nor synergised by NADH. NADH supports less than 10% of the NADPH rate of epoxidation. The epoxidase is inhibited by a carbon monoxide/oxygen atmosphere, half-maximal inhibition occurring at a CO/O₂ ratio of 4.0. This inhibition is reversed by white-light irradiation.