Effects of noradrenaline and N6,O2'-dibutyryl 3',5'-cyclic AMP on adipocyte glycerolipid-synthesizing enzymes

Effect of glutathione deficiency on the adipocyte Mg(2+)-dependent phosphatidate phosphohydrolase

Previous studies from this laboratory [Jamdar S. C. and Cao W. F. (1994) Biochem. J. 301, 793-799] show that the adipocyte Mg(2+)-dependent phosphatidate phosphohydrolase (MGPPH), a major regulatory enzyme in adipose triacylglycerol metabolism, requires an active thiol group for its activity and perturbation of this group results in the loss of enzyme activity. Since glutathione (GSH) is important in maintaining the intracellular thiol state, we have used GSH-deficient animals and adipocytes to test the possibility that intracellular GSH concentration is critical in controlling the MGPPH activity. The MGPPH was measured in the presence of aqueous dispersed phosphatidate, and the release of P1 was taken as a measure of enzyme activity. The GSH deficiency in animals and isolated adipocytes was produced in the presence of diethylmaleate (DEM) or buthionine sulfoximine (BSO). Intraperitoneal administration of BSO into animals (3 mmoles/kg) showed 10-25% reduction in the blood and adipose GSH and 25% decline in the adipose MGPPH activity. However, DEM (0.3 ml/kg) was more effective and caused over 70% reduction of the blood and adipose tissue GSH content and 75% decline in the adipose MGPPH activity within 4 hr of drug administration. After 24 hr, these values returned to normal. Adipocytes incubated with 2.5 mM DEM for 60 min at 37 degrees C also showed a significant reduction in the GSH content and the MGPPH activity present in the cytosol and membrane fractions. The loss of membrane MGPPH was associated with decreased rates of triacylglycerol formation from [14C]palmitate. Pre-incubation of adipocyte homogenates with 1 mM DEM also resulted in > 90% decline in the MGPPH activity, which was preventable in the presence of GSH and dithiothreitol. Therefore, these studies suggest that the sulfhydryl environment offered by glutathione is critical for the maintenance of adipocyte MGPPH activity.

Adipose-tissue Mg2+-dependent phosphatidate phosphohydrolase. Control of activity and subcellular distribution in vitro and in vivo

The subcellular distribution of Mg2+-dependent phosphatidate phosphohydrolase in rat adipocytes between a soluble and a membrane-bound fraction was measured by using both centrifugal fractionation and a novel Millipore-filtration method. The relative proportion of the phosphohydrolase associated with the particulate fraction was increased on incubation of cells with noradrenaline or palmitate. Insulin on its own decreased the proportion of the phosphohydrolase that was particulate and abolished the effect of noradrenaline, but not that of palmitate. The effect of noradrenaline on phosphohydrolase distribution was rapid, the effect being maximal within 10 min. Noradrenaline exerted this effect with a similar concentration-dependence to its lipolytic effect. Inclusion of albumin in homogenization buffers decreased the proportion of the phosphohydrolase that was particulate, but did not abolish the effect of noradrenaline. There was limited correlation between the proportion of the phosphohydrolase that was particulate and the measured rate of triacylglycerol synthesis in adipocytes incubated under a variety of conditions. Starvation, streptozotocin-diabetes and hypothyroidism decreased the specific activities of the phosphohydrolase and glycerolphosphate acyltransferase in homogenates from epididymal fat-pads. Restoration of these activities in the diabetic state was seen after administration of insulin over 2 days or, in the short term, within 2 h after a single administration of insulin. Administration of thyroxine over 3 days caused restoration of these activities in the hypothyroid state. Starvation and diabetes increased the proportion of the phosphohydrolase found in the microsomal fraction. This change was not seen when albumin was present in homogenization buffers. The possible role of fatty acids as regulators of the intracellular translocation of the phosphohydrolase, together with the role of this enzyme in the regulation of triacylglycerol synthesis in adipose tissue, is discussed.