Michaelis-Menten kinetics of galactose elimination by the isolated perfused pig liver

The relation between galactose elimination rates and blood concentrations in the isolated perfused pig liver was analyzed by a mathematical kinetic model. It assumes that the substrate, under steady-state conditions, is removed from the blood that flows through the sinusoids by an irreversible process which follows Michaelis-Menten (i.e., saturation) kinetics. The experiments consisted of successive periods with constant infusions of galactose. The model fitted the data to within the experimental uncertainty. The estimated maximal rate (Vmax) ranged from 0.34 to 0.57 mmol-min(-1)-kg(-1) liver, and the Michaelis constant, Km, ranged from 0.12 to 0.30 mmol-liter(-1) plasma water in nine experiments. The ratio between the galactose concentration in hepatocyte water and plasma water was not significantly different from 1.0, indicating that membrane transport is not rate limiting for the elimination of galactose. In experiments with increasing concentrations of galactose in hepatocyte water and approximately saturated elimination rates, the concentrations of galactose 1-phosphate, UDPgalactose, and UDPglucose remained essentially constant. This indicates that the phosphorylation of galactose to galactose 1-phosphate is the rate-determining process.