Comparative effects of the azole-based fungicide flusilazole on yeast and mammalian lanosterol 14 alpha-methyl demethylase.

Flusilazole inhibits the 14 alpha-demethylation of [24,25-3H-2]dihydrolanosterol by yeast and rat liver cell-free preparations. The 50% inhibitory concentration of demethylation shows that the yeast system is 100 times more sensitive than the rat system. Purified rat liver P-45014DM is about 10 times more sensitive to flusilazole than crude rat liver microsomes. Binding constants in microsomes indicate that yeast preparations (Kd, 21 nM) bind flusilazole with a higher affinity than rat liver (Kd, 496 nM). Purified rat liver P-45014DM (Kd, 45 nM) binds flusilazole with an affinity similar to that of the yeast enzyme. The P-450-dependent cholesterol 7 alpha-hydroxylase in rat liver microsomes is more sensitive to flusilazole than P-45014DM. The results indicate that selectivity of azole antimycotics is not due to inherent sensitivity differences between fungal and mammalian 14 alpha-demethylase; rather, in crude enzyme systems, a protective effect is afforded by other susceptible isozymes present in mammalian systems.