Hormonal regulation of fatty acid synthetase in chick embryo liver

Effect of the peroxisomal proliferator di(2-ethylhexyl)phthalate on component reactions of the rat hepatic microsomal fatty acid chain elongation system and on other hepatic lipogenic enzymes

The feeding of 2% di(2-ethylhexyl)phthalate (DEHP) to rats increased the hepatic microsomal elongation of palmitoyl-CoA by about twofold, while those of palmitoleoyl-CoA and gamma-linolenoyl-CoA decreased to 83 and 63%, respectively, of the control values. When component reactions of the elongation pathway were measured, it was observed that only the activity of condensing enzyme was increased by twofold, while those of beta-ketostearoyl-CoA reductase, beta-hydroxypalmitoyl-CoA dehydrase, and trans-2-hexadecenoyl-CoA reductases were not affected. Furthermore, the time course for induction of both condensation and elongation of palmitoyl-CoA was similar. In vitro addition of DEHP had no effect on either condensation or elongation. Thus, these results indicate that the peroxisomal proliferator induces only the condensing enzyme which is the regulatory and rate-limiting step of elongation sequence. The DEHP treatment also markedly enhanced the cytosolic NADPH-generating activities of glucose-6-PO4 dehydrogenase (2.2-fold) and malic enzyme (7.3-fold). Unexpectedly, the activities of fatty acid synthetase and citrate cleavage enzyme were unaffected. These results are discussed in light of the fact that these lipogenic enzymes are coordinately induced by diet or hormones.

Colchicine inhibition of insulin induction of stearoyl-CoA desaturase and fatty acid synthetase in cultured avian liver explants

Chicken embryo liver explants cultured in chemically defined medium in the absence of serum provide a unique system to probe into the mechanism of insulin induction of lipogenic enzymes. Colchicine at concentrations of 0.2 and 1 microM in the culture medium caused inhibition of insulin induction of stearoyl-CoA desaturase and fatty acid synthetase by 50 and 90%, respectively. As measured by immunochemical techniques, the inhibition of the induction of these two enzyme systems resulted from the decreased content of the delta 9-terminal desaturase component of the stearoyl-CoA desaturase and the fatty acid synthetase. Colchicine, however, had no effect on the general protein synthesis, nor did it affect the malic enzyme, which is induced by triiodothyronine but not by insulin. Also, colchicine had no influence on the binding of 125I-insulin to isolated plasma membrane. Pretreatment of liver explants with insulin for 0.5-1 h and subsequent incubation in insulin-free media for 48 h resulted in induction of the desaturase and fatty acid synthetase. However, inclusion of colchicine in the media for 3 h subsequent to the treatment with insulin completely abolished the inductive effect of insulin, suggesting that colchicine affects events occurring subsequent to insulin binding to the cell surface membranes. Since lumicolchicine, an inactive isomer of colchicine, had no effect on insulin action, it is suggested that the inhibition of insulin induction of the desaturase and synthetase is related to the depolymerizing action of colchicine. Therefore, in eliciting long-term responses to insulin, microtubular integrity of the cell may be required for the transfer of a putative from cell surface insulin receptor to intracellular sites.

Lipogenic enzymes in rat maternal adipose tissue in the perinatal period

1. The specific activities of fatty acid synthetase, acetyl-CoA carboxylase and pyruvate dehydrogenase were measured in rat adipose-tissue extracts in pregnancy and lactation. Fatty acid synthetase specific activity correlates very closely with the rate of fatty acid synthesis, the enzyme specific activity decreasing after mid-pregnancy in a manner very similar to the rate of fatty acid synthesis. Acetyl-CoA carboxylase specific activity also decreases dramatically after mid-pregnancy. Initial pyruvate dehydrogenase specific activity shows a decrease between 2 days pre partum and 2 days post partum, but total enzyme activity shows no significant change in the same period. 2. Immunotitrations of fatty acid synthetase and pyruvate dehydrogenase activities were carried out; the titrations showed that the change in the fatty acid synthetase activity is due to a change in the enzyme amount; the amount of pyruvate dyhydrogenase does not change. Therefore the decrease in fatty acid biosynthesis in subcutaneous and parametrial adipose tissue in late pregnancy and early lactation is associated with a decrease in the amount of at least one of the enzymes involved in fatty acid biosynthesis. The correlation of these events with known hormonal changes is discussed.