Cytochrome P-450-dependent 14α-demethylation of lanosterol in Candida albicans

Abstract

A novel assay for cytochrome P-450-dependent 14α-sterol demethylase of the important opportunistic fungal pathogen, Candida albicans, is described. The enzyme was assayed in microsomal preparations (microsomes) by measuring the incorporation of [14C]lanosterol into (4,14)-desmethylated sterols. The efficacy of different cell-breakage methods was compared; desmethylated-sterol biosynthesis was maximal when cells were broken with a Braun disintegrator. The solubilization of [14C]lanosterol with detergent in the assay system was essential for enzyme activity, which was enhanced considerably when microsomes were gassed with O2. Under those conditions, there was a reciprocal relationship between the amount of radioactivity incorporated into desmethylated sterols and that lost from lanosterol. The major radiolabelled desmethylated sterol was ergosterol. The enzyme had an apparent K(m) of 52.73 ± 2.80 μM and an apparent V(max.) of 0.84 ± 0.14 nmol/min per mg of protein (n = 3). Enzyme activity was decreased greatly when microsomes were treated with CO or the triazole antifungal ICI 153066.