Latency studies on rat liver microsomal glucose-6-phosphatase. Correlation of membrane modification and solubilization by Triton X-114 with the enzymatic activity

Interrelationships between the catalytic properties of glucose-6-phosphatase and the membrane structure of rat liver microsomes were investigated. (1) Membrane modification and solubilization employing the nonionic surfactant Triton X-114 were standardized and analysed by ultracentrifugation, surface tension- and turbidity measurements. (2) The effect of Triton X-114 on the glucose-6-phosphatase activity was studied systematically and the whole magnitude of time- and temperature-dependent inactivation of this enzyme has been demonstrated. The results show that the activity measured is always a resultant of two processes, the beginning of inactivation and the release of latency. Maximal activation of about 600% (83% of apparent latency) was obtained at 0°C. (3) A correlation between membrane modification and solubilization and the conditions under preincubation and test incubation reveals that studies on detergent-disrupted microsomes are performed on structures reassembled from solubilizates and this implies a modified microenvironment in the reconstitutes. (4) Kinetic analyses suggest interrelationships between Triton X-114 and the permeability barrier of the glucose-6-phosphatase system. (5) At 0°C 2-propanol and ethanol are more potent tools for membrane modification than Triton X-114 and release 88% and 86% latent activity corresponding to an activation of the glucose-6-phosphatase of about 850% and 700%, respectively. These observations suggest that detergent treatment of microsomes could not preserve the functional integrity of the glucose-6-phosphate phosphohydrolase, which is one dogma of the substrate-transport hypothesis developed by Arion and his co-workers (Arion, W.J., et al. (1975) Mol. Cell. Biochem. 6, 75–83).