Evidence for a membrane exchangeable glucose pool in the functioning of rat liver glucose-6-phosphatase.
We have investigated the kinetics of tracer uptake into rat liver microsomes in relation to [14C]glucose 6-phosphate (Glu-6-P) hydrolysis by glucose 6-phosphatase (Glu-6-Pase). 1) The steady-state levels of intravesicular tracer accumulated during the rapid (AMP1) and slow (AMP2) phases of uptake both demonstrate Michaelis-Menten kinetics relative to outside Glu-6-P concentrations with Km values similar to those observed for the initial burst (Vi) and steady-state (VSS) rates of Glu-6-P hydrolysis. 2) The AMP1/AMP2 ratio is constant (mean value = 0.105 +/- 0.018) over the whole range of outside Glu-6-P concentrations and is equal to the AMP1max/AMP2max ratio (0.109 +/- 0.032). 3) Linear relationships are observed between the initial rates of glucose transport during the slow uptake phase (V alpha 2) and [AMP1], and between [VSS] and [AMP2]. 4) The value of Vss max exceeds by more than 10-fold that of V alpha 2 max. 5) It is concluded that the substrate transport model is incompatible with those results and that AMP1 represents a membrane exchangeable glucose pool. 6) We propose a new version of the conformational model in which the catalytic site lies deep within a hydrophilic pocket of an intrinsic membrane protein and communicates with the extra- and intravesicular spaces through channels with different glucose permeabilities.
Berteloot
A
A
Department of Nutrition, Université de Montréal, Québec, Canada.
St-Denis
J F
JF
van de Werve
G
G
D000818:Animals; D002462:Cell Membrane / Q000378:metabolism; D002463:Cell Membrane Permeability; D005947:Glucose / Q000378:metabolism*; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D019298:Glucose-6-Phosphate; D005958:Glucosephosphates / Q000378:metabolism*; D006868:Hydrolysis; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D005958:Glucosephosphates; D019298:Glucose-6-Phosphate; D005952:Glucose-6-Phosphatase; D005947:Glucose