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 [C-14]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 (AMP(1)) and slow (AMP(2)) phases of uptake both demonstrate Michaelis-Menten kinetics relative to outside Glu-6-P concentrations with K-m values similar to those observed for the initial burst (V-i) and steady-state (V-ss) rates of Glu-6-P hydrolysis. 2) The AMP(1)/AMP(2) ratio is constant (mean value = 0.105 +/- 0.018) over the whole range of outside Glu-6-P concentrations and is equal to the AMP(1max)/AMP(2max) ratio (0.109 +/- 0.032), 3) Linear relationships are observed between the initial rates of glucose transport during the slow uptake phase (V-a2) and [AMP(1)], and between [V-ss] and [AMP(2)]. 4) The value of V-ss max exceeds by more than 10-fold that of V-a2 max. 5) It is concluded that the substrate transport model is incompatible with those results and that AMP, 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.