Effects of diet on hepatic triglyceride synthesis

Abstract

The effects of diet on the rate of triglyceride synthesis by rat liver homogenates was measured. Changes in triglyceride synthesis were correlated with the level of activity of L-aglycerophosphate acyltransferase, the enzyme catalyzing the first specific reaction in hepatic glycerolipid synthesis.Fasting for 48-72 hr depressed the synthesis of triglyceride from L-a-glycerophosphate. High carbohydrate diets, fed to rats for 6 days, resulted in increased triglyceride synthesis. Diets high in starch were less effective than high glucose, sucrose, or fructose diets in increasing triglyceride synthesis. Diets high in corn oil did not alter triglyceride synthesis. These studies established the importance of dietary factors in the regulation of hepatic triglyceride synthesis.L-a-GlycerophOsphate acyltransferase activity was measured after the same dietary changes. Both high carbohydrate and high fat diets resulted in increased enzyme specific activity. Fasting for 72 hr did not decrease activity. Thus, the specific activity of this enzyme did not correlate well with the measured rate of triglyceride synthesis indicating that other factors must participate in the regulation of triglyceride biosynthesis.

The effect of chronic diabetes, induced by streptozotocin, on the activities of some enzymes of glycerolipid synthesis in rat liver

“…The preference for fatty acid oxidation versus esterification to triacylglycerol is especially well observed in starvation (Mayes & Felts, 1967;Imazumi et al, 1973;Mayes, 1976;Iritani et al, 1976;Groener & van Golde, 1977). A decreased rate of triacylglycerol synthesis has been reported with cell-free preparations obtained from the livers of starved rats (Rubenstein & Rubenstein, 1966;Fallon & Kemp, 1968). The hepatic activity of glycerol phosphate acyltransferase is also decreased in this condition (Vavrecka et al, 1969;Aas & Daae, 1971;Mangiapane et al, 1973;van Tol, 1974) or shows no change (Fallon & Kemp, 1968;Wiegand et al, 1973).…”

“…A decreased rate of triacylglycerol synthesis has been reported with cell-free preparations obtained from the livers of starved rats (Rubenstein & Rubenstein, 1966;Fallon & Kemp, 1968). The hepatic activity of glycerol phosphate acyltransferase is also decreased in this condition (Vavrecka et al, 1969;Aas & Daae, 1971;Mangiapane et al, 1973;van Tol, 1974) or shows no change (Fallon & Kemp, 1968;Wiegand et al, 1973). A lower activity of glycerol phosphate acyltransferase may be partly responsible for enhancing the rate of fatty acid oxidation as opposed to their esterification.…”

“…A lower activity of glycerol phosphate acyltransferase may be partly responsible for enhancing the rate of fatty acid oxidation as opposed to their esterification. However, this enzyme does not appear to be ratelimiting in triacylglycerol synthesis in vitro and its activity correlates poorly with the observed rates of triacylglycerol synthesis (Hubscher et al, 1967;Fallon & Kemp, 1968;Vavrecka et al, 1969;Mangiapane et al, 1973;Fallon et al, 1977;. The enzymic capacity to synthesize triacylglycerols (as opposed to phosphatidate) is maintained, or even increased, in starvation provided that the assays are performed in the presence of optimum concentrations of cofactors (Rubenstein & Rubenstein, 1966;Vavrecka et al, 1969;Mangiapane et al, 1973).…”

“…The activities of the total microsomal acyl-CoA synthetase (EC 6.2.1.3) and of glycerol phosphate acyltransferase do respond to some physiological stimuli, but the activities do not always correlate well with the overall rates of triacylglycerol synthesis, and the changes, when observed at all, are usually smaller than those for the soluble phosphatidate phosphohydrolase (Fallon & Kemp, 1968;Vavrecka et al, 1969;Aas & Daae, 1971;Mangiapane et al, 1973;Wiegand et al, 1973;Brandes et al, 1973;Lamb & Fallon, 1974;Fallon et al, 1977;; Table 2). The activity of the soluble phosphatidate phosphohydrolase can change rapidly in response to physiological stimuli and parallels the altered rate of triacylglycerol synthesis (Mangiapane et al, 1973;Lamb & Fallon, 1974;Fallon et al, 1977;.…”

The Beriberi Heart

“…Recent evidence points to a prominent role of the hypothalamus in triglyceride metabolism through sympathetic nervous system action (11). High carbohydrate diets fed to rats for six days resulted in increased triglyceride synthesis, establishing the importance of dietary factors in the regulation of hepatic triglyceride synthesis (12).…”