Hysteresis at near-physiologic substrate concentrations underlies apparent sigmoid kinetics of the glucose-6-phosphatase system.

Although Canfield and Arion (J. Biol. Chem. 263, 7458-7460 (1990)) have described the kinetics as hyperbolic, Traxinger and Nordlie (J. Biol. Chem. 262, 10015-10019 (1987)) reported sigmoid kinetics in the glucose-6-phosphatase system of intact microsomes at near-physiologic glucose-6-P concentrations. We show here that apparent sigmoidal kinetics, most clearly seen as sharp upward inflections in Hanes plots as substrate concentration approaches zero, are a consequence of the hysteretic lag in product formation during the first minutes of incubation of the enzyme with low concentrations of substrate. The appearance of sigmoidicity, observed when reaction velocities are calculated from changes in Pi concentration between 0 and 6 min of incubation, is not present when velocity is determined from slopes of [product]-time plots after linearity is achieved. The Km,glucose-6-P value, 0.86 mM, based on these hysteresis-corrected velocity values determined with intact microsomes from normal, control rats at low substrate concentrations, approached the upper limit of physiologic hepatic glucose-6-P concentrations. This suggests that glucose-6-phosphatase activity may be regulated by factors other than substrate concentrations alone. We propose that the hysteretic behavior, not sigmoid kinetics of the glucose-6-phosphatase enzyme system, may be a prime regulatory feature.