Centrifugations ofHomogenates from Glucose-Repressed Saccharomyces cerevisiae By TIMO NURMINEN, LEO TASKINEN and HEIKKI SUOMALAINEN

1. The distributions ofseveral enzymes and other marker components were examined after zonal centrifugations of whole homogenates from glucose-repressed Saccharomyces cerevisiae on sucrose and iso-osmotic Ficoll, and the composition and morphology ofthe fractions were investigated. 2. After high-speed zonal centrifugation most of the protein, acid and alkaline phosphatases, alkaline pyrophosphatase, adenosine monophosphatase, ,8-fructofuranosidase, a-mannosidase, NADPH-cytochrome c oxidoreductase and an appreciable amount of phospholipid and sterol were non-sedimentable, i.e. were at densities below 1.09 (g/cm3). Most of the RNA was at p = 1.06-1.08 in Ficoll and at p =1.09-1.11 in sucrose. 3. The bulk of the Mg2+-dependent adenosine triphosphatase (Mg-ATPase) was coincident with the main peak of phospholipid and sterol, at median density 1.10, which was also rich in smooth-membrane vesicles. In Ficoll, a minor peak of phospholipid and sterol at p 1.12-1.15 contained a smaller part of the oligomycininsensitive Mg-ATPase and heavy membrane fragments. In sucrose, several minor peaks of Mg-ATPase were in the mitochondrial density range, and a peak of oligomycininsensitive Mg-ATPase coincident with a minor peak of phospholipid and sterol at around p 1.25 contained heavy membrane fragments of high carbohydrate content, especially mannose. 4. Further purification of the oligomycin-insensitive Mg-ATPase containing membrane preparations was performed on Urografin gradients. 5. It is argued that the oligomycin-insensitive Mg-ATPase containing membranes are fragments of the plasma membrane, but have different densities because they contain different amounts of glycoprotein particles.