Role of glycerol 3-phosphate and glycerophosphate acyltransferase in the nutritional control of hepatic triacylglycerol synthesis

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Abstract
1. Glycerol 3-phosphate content of isolated hepatocytes from starved rats and of glycogen-depleted hepatocytes from fed rats was low and severely limited triacylglycerol synthesis. 2. Raising the glycerol 3-phosphate content by addition of precursors to the cells resulted in a hyperbolic-like relationship between triacylglycerol synthesis and cellular glycerol 3-phosphate content. Statistical analysis of the curves showed no significant differences between the nutritional states either at saturating or at subsaturating glycerol 3-phosphate content. 3. Vmax. of glycerophosphate acyltransferase measured in homogenized hepatocytes was decreased by 30–40% in starvation. There was no change in apparent Km for glycerol 3-phosphate. Since at saturating glycerol 3-phosphate content esterification rates in hepatocytes of both nutritional states were identical, the enzyme is not limiting esterification under this condition. 4. At subsaturating glycerol 3-phosphate content the flux through glycerophosphate acyltransferase necessarily limits esterification. Therefore one would expect a decrease in esterification in starvation under this condition. This was the case when triacylglycerol synthesis was plotted against intracellular glycerol 3-phosphate concentration, calculated from the cellular glycerol 3-phosphate content and the intracellular water space, which was smaller in hepatocytes from starved rats. 5. The data obtained in hepatocytes were extrapolated to the intact liver by using the number of parenchymal cells per g of liver as determined from marker-enzyme analysis and the liver weight per 100g body weight. The extrapolation suggested that glycerol 3-phosphate is limiting esterification in vivo for contents below 0.3–0.4 and 0.5–0.65μmol/g for livers from fed and starved animals respectively. Also for a given fatty acid load and a glycerol 3-phosphate content below 0.3μmol/g the liver may esterify less in the starved state. However, at the glycerol 3-phosphate contents measured in freeze-clamped livers (0.30 and 0.44μmol/g for the fed and starved state respectively), livers in both nutritional states seemed capable of esterifying similar amounts of fatty acids.

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