Calcium sequestration activity in rat liver microsomes. Evidence for a cooperation of calcium transport with glucose-6-phosphatase.
Mechanisms regulating the energy-dependent calcium sequestering activity of liver microsomes were studied. The possibility for a physiologic mechanism capable of entrapping the transported Ca2+ was investigated. It was found that the addition of glucose 6-phosphate to the incubation system for MgATP-dependent microsomal calcium transport results in a marked stimulation of Ca2+ uptake. The uptake at 30 min is about 50% of that obtained with oxalate when the incubation is carried out at pH 6.8, which is the pH optimum for oxalate-stimulated calcium uptake. However, at physiological pH values (7.2-7.4), the glucose 6-phosphate-stimulated calcium uptake is maximal and equals that obtained with oxalate at pH 6.8. The Vmax of the glucose 6-phosphate-stimulated transport is 22.3 nmol of calcium/mg protein per min. The apparent Km for calcium calculated from total calcium concentrations is 31.9 microM. After the incubation of the system for MgATP-dependent microsomal calcium transport in the presence of glucose 6-phosphate, inorganic phosphorus and calcium are found in equal concentrations, on a molar base, in the recovered microsomal fraction. In the system for the glucose 6-phosphate-stimulated calcium uptake, glucose 6-phosphate is actively hydrolyzed by the glucose-6-phosphatase activity of liver microsomes. The latter activity is not influenced by concomitant calcium uptake. Calcium uptake is maximal when the concentration of glucose 6-phosphate in the system is 1-3 mM, which is much lower than that necessary to saturate glucose-6-phosphatase. These results are interpreted in the light of a possible cooperative activity between the energy-dependent calcium pump of liver microsomes and the glucose-6-phosphatase multicomponent system. The physiological implications of such a cooperation are discussed.
Benedetti
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A
Fulceri
R
R
Comporti
M
M
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D010070:Oxalates / Q000494:pharmacology; D019815:Oxalic Acid; D010710:Phosphates / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains
D005958:Glucosephosphates; D010070:Oxalates; D010710:Phosphates; D019298:Glucose-6-Phosphate; D000255:Adenosine Triphosphate; D019815:Oxalic Acid; D005952:Glucose-6-Phosphatase; D002118:Calcium