Control of Lipid Metabolism in Adipose‐Tissue Hornogenates of Fasted Refed Rats

Abstract

1.The 700 x g fat-poor internatant from epididymal adipose tissue homogenate of fasted refed rats was incubated a t pH6.5 with 0.5mM [1-l4C]acetate in the presence of cofactors. After incubation, six lipid classes were separated by thin-layer chromatography and their fatty acids analyzed by radio gas-liquid chromatography. The incorporation of [ 1 -14C]acetate into glycerides was limited by the activity of acetyl-CoA carboxylase, and was stimulated two to three-fold by the addition of 0.1 mM unlabelled malonyl-CoA.2. The chain-length of fatty acids labelled in vitro was controlled by the relative concentrations of malonyl-CoA and acetyl-CoA. Addition of unlabelled malonyl-CoA influenced the operation of fatty acid synthetase in such a way that the ratio of radioactive palmitate to radioactive myristate increased markedly.3. sn-Glycerol 3-phosphate stimulated the esterification of fatty acids into triglycerides. Under these conditions whereas the specific activity of free fatty acids was much higher than that observed in triglycerides, the specific activity of diglycerides was low, when compared to the same triglycerides. Newly synthesized monounsaturated fatty acids were preferentially esterified into triglycerides. 0.5 mM [1-14C]palmitate was efficiently incorporated into triglycerides but not into diglycerides.Given any cell size and cellular proliferation rate, the intermediary metabolism of adipocytes is regulated by the concentration and activity of proteins, the levels of metabolites and by hormones. After having explored lipid metabolism in epididymal adipose tissue fragments and isolated fat cells [1,2], an investigation was made of de novo fatty acid and glyceride synthesis in 700 x g adipose tissue internatants. This system includes "natura1" proportions of both soluble proteins and the proteins of several adipose cell organelles (mitochondria, microsomes and liposomes). The conditions were aimed at testing the activity of acetyl-CoA synthetase, acyl-CoA synthetase, -dehydrogenase and -elongase, and the esterifying enzymes of Kennedy's pathway. In some instances, acetyl-CoA carboxylase was also investigated.Detailed studies of the activity of fatty acid synthetase have been reported in liver [3], mammary Enzymes.

Desaturation of stearic acid by liver and adipose tissue from obese-hyperglycaemic mice (ob/ob)

“…The results of the direct assay of stearic acid desaturase reported in this paper demonstrate that the increased incorporation of precursors into palmitoleic acid and oleic acid observed by Christophe's group in whole-tissue preparations from obese mice are due, at least in part, to an increase in the fatty acid desaturase system (Winand et al, 1968(Winand et al, , 1969a(Winand et al, , 1973. The threefold increase in the activity of stearic acid desaturase on a protein basis is similar to the increase in citrate cleavage enzyme (Komacker & Lowenstein, 1964) acetyl-CoA carboxylase and fatty acid synthetase (Chang et al, 1967) in livers from 12-week-old obese-hyperglycaemic mice compared with lean mice.…”

“…The only consistent change in fatty acid composition, however, is an increase in the proportion of palmitoleic acid (Haessler & Crawford, 1965;Stein et al, 1967;Winand et al, 1969b;Bullfield, 1972) with a compensatory small decrease in several fatty acids but mainly linoleic acid. The obese strains show both a high capacity to deposit dietary fat (Lemonnier et al, 1971) and a higher lipogenic capacity (for a review see Bray & York, 1971), which may maintain the fatty acid composition near normal since mouse diets usually contain a lower proportion of oleic acid than linoleic acid, the reverse of the proportions in adipose tissue.…”

“…Obese mice of the ob/ob strain are hyperinsulinaemic (Stauffacher et al, 1967), and this together with the increase in palmitoleic acid indicates that they might have an enhanced fatty acid desaturase activity. Indeed Christophe and his colleagues (Winand et al, 1968(Winand et al, , 1969a(Winand et al, ,b, 1973 have observed an increased conversion of glucose, acetic acid and palmitic acid into palmitoleic acid and oleic Vol. 148 acid both in vivo and in vitro in whole-tissue preparations from obese mice compared with their lean controls.…”

The role of insulin in the regulation of stearic acid desaturase activity in liver and adipose tissue from obese–hyperglycaemic (ob/ob) and lean mice

“…Injection of insulin also increases the activity in livers of normal rats (Inkpen et al, 1969). Increased synthesis of palmitoleic acid and oleic acid has been demonstrated in the liver and adipose tissue of obese-hyperglycaemic (ob/ob) mice (Winand et al, 1968(Winand et al, , 1969a(Winand et al, ,b, 1973 and it has been confirmed that this occurs through an increase in the activity of the desaturase enzyme system (Enser, 1975). Since obese mice are markedly hyperinsulinaemic (Stauffacher et al, 1967;Christophe et al, 1959) this seemed a likely cause of the increased desaturase activity.…”

“…Polyunsaturated fatty acids decrease and saturated fatty acids increase the desaturase activity. Since the carcass lipids of obese mice contain lower proportions of polyunsaturated fatty acids than lean mice (Haessler & Crawford, 1965;Stein et al, 1967;Winand et al, 1969b;Bullfield, 1972), the increased desaturase activity might be explained by a decrease in their inhibitory activity. Food intake also regulates A9-desaturase activity (Inkpen et al, 1969;Lee & Sprecher, 1971) with a decrease during starvation and an increase to Vol.…”