The 3-hydroxy-3-methylglutaryl-coenzyme A reductase of Aspergillus nidulans

Abstract

The 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR) of the filamentous fungus Aspergillus nidulans has been analyzed to further characterize the regulatory aspects of ergosterol biosynthesis. The supernatant contained approximately 95% of the total recovered activity following centrifugation (8000g) of homogenized cells. Following ultracentrifugation (105, 000g), total activity was evenly distributed between the supernatant and pellet, although specific activity was 2.5-fold higher in the pellet. The enzyme possessed a pH optimum at pH 7.0, along with a second peak at pH 5.5. The V max was 328 pmol NADPH oxidized per minute, and the apparent K m for dl-HMG-CoA was 28.5 μ M. Consistent with the pattern of sterol production, HMGR specific activity was highest during rapid growth and decreased during stationary phase. This pattern is different from that observed in Saccharomyces cerevisiae and suggests that the regulation of ergosterol biosynthesis in A. nidulans is quite different from that observed in yeast. Addition of exogenous ergosterol (1–10 μg/ml) to the medium (in the presence of the sterol synthesis inhibitor micronazole) caused at 31–36% decrease in HMGR specific activity in culture extracts. This reaction to ergosterol (or a metabolic by-product) suggested that A. nidulans may utilize a regulatory response analogous to that reported in animal cells.