Abstract
The relationships between the physical state of membrane lipids and enzymatic or transport activities have been analyzed in several publications [l-5] using various microorganisms. Discontinuities in the activation energies of enzymatic activities or transport processes were found to depend on the fatty acid composition and cholesterol content of membranes. Such discontinuities sometimes coincided with the phase transition of lipids as detected by X-ray diffraction or thermal analysis [3,5] . However, sometimes disparities between the activity transition temperatures and lipid transition temperatures were observed [ 1 ] . Analogous discrepancies have been seen in studies with isolated mitochondria. Different activities of membrane-bound enzymes of mitochondria exhibit breaks in their Arrhenius plots at 23-24°C and at lo12°C in homeothermic animals and chilling-sensitive plants, respectively [6] . Electron spin resonance experiments showed that the temperatures of the breaks in the Arrhenius plots of some enzymatic activities in rat liver mitochondria coin-
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