Ecdysone 20-hydroxylation inManduca sexta midgut: Kinetic parameters of mitochondrial and microsomal ecdysone 20-monooxygenases

Abstract

The larval midgut of the tobacco hornworm, Manduca sexta, has high ecdysone 20-monooxygenase (E20MO) activity, located both in the mitochondria and in the microsomes. The apparent kinetic parameters for E20MO in mitochondria and microsomes were determined. The Km5 (for ecdysone) of the mitochondrial and microsomal enzymes were 1.63 × 10−5 and 3.67 × 10−7 M, respectively. The Vmax was 82.7 pmol/min/mg protein for mitochondria and 32.0 pmol/min/mg protein for microsomes. Although the mitochondrial E20MO has the higher Vmax, at physiological ecdysone concentrations (10−7 − 10−8 M) it is only one-eighth to one-tenth as active as the microsomal enzyme. It is concluded that the microsomal E20MO is the primary, if not the only, enzyme involved in ecdysone 20-hydroxylation in M. sexta midgut. © 1996 Wiley-Liss, Inc. This article is a US Government work and, as such, is in the public domain in the United States of America.