title: Microsomal membrane permeability and the hepatic glucose-6-phosphatase system. Interactions of the system with D-mannose 6-phosphate and D-mannose.
abstract: We have proposed that glucose-6-phosphatase (EC 3.1.3.9) is a two-component system consisting of (a) a glucose-6-P-specific transporter which mediates the movement of the hexose phosphate from the cytosol to the lumen of the endoplasmic reticulum (or cisternae of the isolated microsomal vesicle), and (b) a nonspecific phosphohydrolase-phosphotransferase localized on the luminal surface of the membrane (Arion, W.J., Wallin, B.K., Lange, A.J., and Ballas, L.M. (1975) Mol. Cell. Biochem. 6, 75-83). Additional support for this model has been obtained by studying the interactions of D-mannose-6-P and D-mannose with the enzyme of untreated (i.e. intact) and taurocholate-disrupted microsomes. An exact correspondence was shown between the mannose-6-P phosphohydrolase activity at low substrate concentrations and the permeability of the microsomal membrane to EDTA. The state of intactness of the membrane influenced the kinetics of mannose inhibition of glucose-6-P hydrolysis; uncompetitive and noncompetitive inhibitions were observed for intact and disrupted microsomes, respectively. The apparent Km for glucose-6-P was smaller with intact preparations at mannose concentrations above 0.3 M. Mannose significantly inhibited total glucose-6-P utilization by intact microsomes, whereas D-glucose had a stimulatory effect. Both hexoses markedly enhanced the rate of glucose-6-P utilization by disrupted microsomes. The actions of mannose on the glucose-6-phosphatase of intact microsomes fully support the postulated transport model. They are predictable consequences of the synthesis and accumulation of mannose-6-P in the cisternae of microsomal vesicles which possess a nonspecific, multifunctional enzyme on the inner surface and a limiting membrane permeable to D-glucose, D-mannose, glucose-6-P, but impermeable to mannose-6-P. The latency of the mannose-6-P phosphohydrolase activity is proposed as a reliable, quantitative index of microsomal membrane integrity. The inherent limitations of the use of EDTA permeability for this purpose are discussed.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active; D005947:Glucose / Q000378:metabolism; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D005958:Glucosephosphates / Q000378:metabolism; D006600:Hexosephosphates / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008358:Mannose / Q000378:metabolism*; D008566:Membranes / Q000187:drug effects / Q000378:metabolism*; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D010539:Permeability; D010770:Phosphotransferases / Q000378:metabolism; D051381:Rats; D013656:Taurocholic Acid / Q000494:pharmacology
chemical_list: D005958:Glucosephosphates; D006600:Hexosephosphates; D013656:Taurocholic Acid; D010770:Phosphotransferases; D005952:Glucose-6-Phosphatase; D005947:Glucose; D008358:Mannose