The effects of ergosterol and alcohols on germ-tube formation and chitin synthase in Candida albicans

Ergosterol inhibited germ-tube formation and the membrane-bound enzyme chitin synthase in Candida albicans. The sterol solvents methanol and ethanol stimulated chitin synthase activity but inhibited germination. Methanol increased both the rate of protease activation of the chitin synthase proenzyme and the Vmax of the enzyme. The activation was not due to a change in the protease action on the proenzyme. The methanol activation was reversible and therefore the extraction of inhibitory components from the membrane was not causing the activation. Methanol had no effect on the N-acetylglucosamine activation sites. Analysis of the N-acetylglucosamine activation by an iteration programme gave Ka values of 0.3 mM for the high affinity sites and 20 mM for the low affinity sites. The degree of cooperativity with respect to the substrate uridine diphosphate N-acetylglucosamine was not changed by the presence of methanol and therefore the solvent did not affect the subunit–subunit interactions of the enzyme. Arrhenius plots of chitin synthase have discontinuities and methanol did not affect the transition temperature indicating that the structure of the membrane in the immediate vicinity of the enzyme was unchanged.