Title: A ROLE FOR STEROLS AS M99AATORS OF MEMBRANE FLUIDITY

The role of sterols as structural components in the mitochondria] membrane of Saccharomyces cerevisiae was investigated. Gel to liquid-crystalline transitions of mitochondria] membrane preparations from mutant and wild type cells were probed using the technique of fluores-cence polarization. Mitochondria isolated from those strains accumula-ting sterols other than ergosterol exhibited a phase transition; wild types did not. The technique was extended to an artificial membrane system using a series of sterols with modifications in the hydroxyl moiety, the ring system, and the side chain to investigate the con-tribution of various sterol structures to membrane fluidity. Sterols possessing a planar ring, a R-hydroxyl and a saturated side chain con-ferred maximal order to the bilayer. Arrhenius kinetics of the mito-chondrial membrane-bound enzyme L-kynurenine-3-hydroxylase paralleled changes in fluidity observed in the fluorescence study. Examination of sphaeroplast and mitochondrial response to osmotic strength showed no correlation between sterol structure and ability to resist stretch-ing. Uptake studies of the lipophilic cation methyltriphenyl-phosphonium by the mitochondria enabled the calculation of the trans-membrane potential. Potentials for mitochondria isolated from organisms accumulating sterols with side chain modifications were ten millivolts lower than those from the wild type strains. No variation of respira-tory competency as a function of sterol composition was discerned. The observations are discussed in relation to an adjustment of the membrane properties to accomodate sterols other than ergosterol without extensive alteration of enzymatic activities and permeability properties. The organisms used were characterized as to sterol to phospholipid ratio and fatty acid composition