Abstract
Analysis of the temperature dependence of the monosaccharide transport system in the yeast Rhodotorula gracilis (ATCC 26194, CBS 6681), as tested with d-xylose, revealed that the apparent affinity of the transport system, measured as the reciprocal of the half-saturation constant K T, increased when transport velocity was stimulated by temperature (15–30°C) and decreased when the rate of uptake was reduced at temperatures above 30°C. Breaks in Arrhenius plots were accompanied by corresponding breaks in van't Hoff plots. Whereas untreated cells exhibited in the van't Hoff plot a discontinuity at 28–30°C this was not observed in heat-treated cells (at either 37 or 45°C). In heattreated cells the maximum transport velocity was always lower and the apparent affinity higher than in untreated cells at the same temperature; the optimum temperature for both transport velocity and apparent affinity was shifted to higher values. The data are interpreted in terms of a reversible phase transition of membrane lipids effecting an irreversible alteration of membrane structure. The temperature-induced reversible alkalinization of unbuffered yeast suspensions supports this interpretation.
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