Oxidation of Glycerol 3-Phosphate by the Perfused Rat Liver

Abstract

Abstract Detritiation of [2-3H]glycerol-3-P by isolated liver mitochondria can be used to assay the activity of mitochondrial glycerol-3-P dehydrogenase. The method differs from measurements of dihydroxyacetone-3-P formation only by the isotope effect on the reaction rate. Under assay conditions liver mitochondria of hyperthyroid rats detritiated [2-3H]glycerol-3-P 16 times faster than those of normal controls. However, the value of K0.5s (the concentration of glycerol-3-P giving 0.5 Vmax) for the oxidation of glycerol-3-P by mitochondria of hyperthyroid rats was 5 times higher than for the reaction catalyzed by mitochondria of normal animals. From the data given by studies with isolated mitochondria, the expected rates of detritiation of [2-3H[glycerol-3-P by the intact liver could be estimated. Experiments then carried out on perfused livers of hypothyroid, normal, and hyperthyroid rats revealed that the rates of the reaction occurring in the intact organ exceeded by far the assayed capabilities of the mitochondrial glycerol-3-P dehydrogenase system. These results indicated that the rate of detritiation of [2-3H]-glycerol-3-P by the liver is not determined by the activity of the mitochondrial enzyme. Although livers of hyperthyroid rats were found to detritiate [2-3H]glycerol-3-P twice as rapidly as those of normal controls, the activity of mitochondrial glycerol-3-P dehydrogenase was far too low to account for the increase. These findings led to a test of the role of the NADH dehydrogenase system in the reaction. Infusion of rotenone into the liver of a normal rat resulted in a marked decrease in the rate of detritiation of [2-3H]glycerol-3-P, the rotenone-insensitive pathway accounting for only 20% of the over-all rate. The results described in this communication do not support the concept of a glycerol-3-P cycle in liver regulated by thyroid hormones but indicate that the pathway of oxidation of extramitochondrial hydrogen leads through respiratory chain-linked NADH dehydrogenase.