STEREOSELECTIVE INTERACTION OF TETRACONAZOLE WITH 14-ALPHA-DEMETHYLASE IN FUNGI

Tetraconazole racemate and its enantiomers were compared for their dif ferent capability to affect growth and ergosterol biosynthesis in Usti lago maydis and Saccharomyces cerevisiae. At subletal doses quantitati ve effects on sterols in sporidia of U. maydis, when expressed as in v ivo IC50 values, were found to coincide with in vitro IC50 values (con centrations which inhibit net ergosterol biosynthesis in cell-free ext ract by 50%). Impairment of cell separation was the early symptom of f ungitoxicity and the related EC(50) values (concentrations which inhib it cell multiplication by 50%) were aligned with the biochemical effec ts. S. cerevisiae was shown to be the less sensitive species of severa l fungi. Sterol biosynthesis inhibition ranked the enantiomers accordi ng to their order of fungitoxicity on both organisms tested. The enant iomeric activity ratios (R/S), when determined by the in vitro IC50 va lues, were in the range of 10-13 in both cases, with the R-(+)-isomer being the most active one. Interaction with cytochrome P450, partially purified from microsomes of S. cerevisiae and utilized as 1.4 x 10(-7 ) M solution, showed type II spectra with saturation reached when appr oximately equimolecular amounts of the separate isomers or racemate we re added. This supports the view that both tetraconazole enantiomers h ave very high affinity to the target site, as also shown with microsom al P450 isolated from U. maydis. The different strength of their bindi ngs to the target site apparently rests on the CO displacement tests, by which the S-(-)-isomer is more easily removed than its optical anti pode. Accordingly, the stereoselective accommodation of dichlorophenyl and tetrafluoroethyl ether moieties of tetraconazole within the activ e site of the target enzyme are briefly discussed. (C) 1995 Academic P ress, Inc.