Udpglucose dehydrogenase Kinetics and their mechanistic implications

Abstract

Initial velocity and product inhibition studies were carried out on UDPglucose dehydrogenase (UDPglucose: NAD + 6-oxidoreductase, EC 1.1.1.22) from beef liver to determine if the kinetics of the reaction are compatible with the established mechanism. An intersecting initial velocity pattern was observed with NAD + as the variable substrate and UDPG as the changing fixed substrate. UDPglucuronic acid gave competitive inhibition of UDPG and non-competitive inhibition of NAD +. Inhibition by NADH gave complex patterns. Lineweaver-Burk plots of 1 υ versus 1/NAD + at varied levels of NADH gave highly non-linear curves. At levels of NAD + below 0.05 mM, non-competitive inhibition patterns were observed giving parabolic curves. Extrapolation to saturation with NAD + showed NADH gave linear uncompetitive inhibition of UDPG if NAD + was saturating. However, at levels of NAD + above 0.10 mM, NADH became a competitive inhibitor of NAD + (parabolic curves) and when NAD + was saturating NADH gave no inhibition of UDPG. NADH was non-competitive versus UDPG when NAD + was not saturating. These results are compatible with a mechanism in which UDPG binds first, followed by NAD +, which is reduced and released. A second mol of NAD + is then bound, reduced, and released. The irreversible step in the reaction must occur after the release of the second mol of NADH but before the release of UDPglucuronic acid. This is apparently caused by the hydrolysis of a thiol ester between UDPglucoronic acid and the essential thiol group of the enzyme. Examination of rate equations indicated that this hydrolysis is the rate-limiting step in the overall reaction. The discontinuity in the velocities observed at high NAD + concentrations is apparently caused by the binding of NAD + in the active site after the release of the second mol of NADH, eliminating the NADH inhibition when NAD + becomes saturating.