HYSTERESIS AT NEAR-PHYSIOLOGICAL SUBSTRATE CONCENTRATIONS UNDERLIES APPARENT SIGMOID KINETICS OF THE GLUCOSE-6-PHOSPHATASE SYSTEM

Although Canfield and Arion (J. Biol. Chem- 263, 7458-7460 (1990)) hav e described the kinetics as hyperbolic, Traxinger and Nordlie (J. Biol . Chem. 262, 10015-10019 (1987)) reported sigmoid kinetics in the gluc ose-6-phosphatase system of intact microsomes at near-physiologic gluc ose-6-P concentrations. We show here that apparent sigmoidal kinetics, most clearly seen as sharp upward inflections in Hanes plots as subst rate concentration approaches zero, are a consequence of the hystereti c lag in product formation during the first minutes of incubation of t he enzyme with low concentrations of substrate. The appearance of sigm oidicity, observed when reaction velocities are calculated from change s in P(i) concentration between 0 and 6 min of incubation, is not pres ent when velocity is determined from slopes of [product]-time plots af ter linearity is achieved. The K(m,glucose-6-P) value, 0.86 mM, based on these hysteresis-corrected velocity values determined with intact m icrosomes from normal, control rats at low substrate concentrations, a pproached the upper limit of physiologic hepatic glucose-6-P concentra tions. This suggests that glucose-6-phosphatase activity may be regula ted by factors other than substrate concentrations alone. We propose t hat the hysteretic behavior, not sigmoid kinetics of the glucose-6-pho sphatase enzyme system, may be a prime regulatory feature.