Direct evidence for the involvement of two glucose 6-phosphate-binding sites in the glucose-6-phosphatase activity of intact liver microsomes. Characterization of T1, the microsomal glucose 6-phosphate transport protein by a direct binding assay.
S 5627 is a synthetic analogue of chlorogenic acid. S 5627 is a potent linear competitive inhibitor of glucose 6-phosphate (Glc-6-P) hydrolysis by intact microsomes (Ki = 41 nM) but is without effect on the enzyme in detergent- or NH4OH-disrupted microsomes. 3H-S 5627 was synthesized and used as a ligand in binding studies directed at characterizing T1, the Glc-6-P transporter. Binding was evaluated using Ca2+-aggregated microsomes, which can be sedimented at low g forces. Aside from a modest reduction in K values for both substrate and S 5627, Ca2+ aggregation had no effect on glucose-6-phosphatase (Glc-6-Pase). Scatchard plots of binding data are readily fit to a simple "two-site" model, with Kd = 21 nM for the high affinity site and Kd = 2 microM for the low affinity site. Binding to the high affinity site was competitively blocked by Glc-6-P (Ki = 9 microM), whereas binding was unaffected by mannose-6-phosphate, Pi, and PPi and only modestly depressed by 2-deoxy-D-glucose 6-phosphate, a poor substrate for Glc-6-Pase in intact microsomes. Thus the high affinity 3H-S 5627 binding site fits the criteria for T1. Permeabilization of the membrane with 0.3% (3-[(chloramidopropyl)-dimethylammonio]-1-propanesulfonate) activated Glc-6-Pase and broadened its substrate specificity, but it did not significantly alter the binding of 3H-S 5627 to the high affinity sites or the ability of Glc-6-P to block binding. These data demonstrate unequivocally that two independent Glc-6-P binding sites are involved in the hydrolysis of Glc-6-P by intact microsomes. The present findings are the strongest and most direct evidence to date against the notion that the substrate specificity and the intrinsic activity of Glc-6-Pase in native membranes are determined by specific conformational constraints imposed on the enzyme protein. These data constitute compelling evidence for the role of T1 in Glc-6-Pase activity.
Arion
W J
WJ
Division of Nutritional Sciences, Cornell University, Ithaca, New York 14853, USA. WJA1@cornell.edu
Canfield
W K
WK
Callaway
E S
ES
Burger
H J
HJ
Hemmerle
H
H
Schubert
G
G
Herling
A W
AW
Oekonomopulos
R
R
D000818:Animals; D001665:Binding Sites; D001667:Binding, Competitive; D002118:Calcium / Q000494:pharmacology; D002726:Chlorogenic Acid / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D002793:Cholic Acids / Q000494:pharmacology; D005952:Glucose-6-Phosphatase / Q000037:antagonists & inhibitors / Q000187:drug effects / Q000378:metabolism*; D019298:Glucose-6-Phosphate / Q000378:metabolism*; D007700:Kinetics; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology*; D008956:Models, Chemical; D051381:Rats; D014316:Tritium
D002793:Cholic Acids; C111165:S 5627; D014316:Tritium; D002726:Chlorogenic Acid; D019298:Glucose-6-Phosphate; D005952:Glucose-6-Phosphatase; C028213:3-((3-cholamidopropyl)dimethylammonium)-1-propanesulfonate; D002118:Calcium