Farnesol oxidation in insects: evidence that the biosynthesis of insect juvenile hormone is mediated by a specific alcohol oxidase

Abstract

The oxidation of farnesol to farnesoic acid is a key step in insect juvenile hormone biosynthesis. We herein present preliminary characterization of the enzyme-catalyzed oxidation of farnesol to farnesal in larval corpora allata homogenates of the tobacco hornworm, Manduca sexta. This conversion, which is highly substrate specific, has a K m apparent of 1 μM and a pH optimum between 6 and 7. Results from chemical modification experiments indicate that the enzyme possesses an active site tyrosine residue. Although farnesol oxidation in adult M. sexta corpora allata homogenates was previously identified as being catalyzed by a dehydrogenase, the corresponding conversion in larvae is not effected by the addition of nicotinamide cofactors. Instead, enzymatic activity is slightly enhanced by the addition of FAD, decreases when incubations are performed anaerobically, and is completely inhibited when either sodium dithionite or glucose oxidase is added. Although the effect of various additives suggests that the oxidation of farnesol to farnesal does not require a metal redox center, 1,10-phenanthroline (but not 4,7-phenanthroline) is a weak irreversible inhibitor of farnesol oxidation (IC 50=11 mM). The addition of exogenous metals (Fe 2+, Cu 2+, Ni 2+, and Co 2+) caused differential effects on farnesol metabolism, with Cu 2+ being highly inhibitory. Taken together, this data suggests that the oxidation of farnesol to farnesal in larval corpora allata is mediated by a specific oxygen-dependent enzyme, perhaps a flavin and/or iron-dependent oxidase.