Awan, Mohammad Moneeb; (1993) The role of malonyl-CoA in the regulation of fatty acid metabolism in cardiac muscle. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Viable, rod-shaped, trypan blue-exclusive cardiac myocytes were purified from collagenase perfused heart. The cells showed typical cross-striations and rhythmical beating movements. This ceil preparation was used to demonstrate the existence of the soluble enzymes ATP-citrate lyase and a citrate-dependent acetyl-CoA carboxylase, together with the membrane-bound condensing enzyme of the fatty acid elongation system. The Km for malonyl-CoA of the fatty acid elongation system was 50μM. Using sensitive radiochemical assay techniques, no fatty acid synthase activity was demonstrable in myocytes. Significant levels of the enzyme were measureable in extracts of whole heart. The myocytes were also used to investigate the carbohydrate-mediated suppression of fatty acid oxidation. Using either 5mM glucose or 2mM lactate, the rate of [1-14C]paimitate was decreased by approximately 40%. In the presence of glucose, insulin (8nM) decreased and adrenaline (5μM) increased the rate of palmitate oxidation by myocytes (30% and 100% respectively). With glucose present 5μM phenylephrine decreased palmitate oxidation by 23%, and 5μM isoprenaline increased palmitate oxidation by 40%. Insulin and adrenaline had no significant effect in the absence of glucose. The levels of malonyl-CoA, long-chain acyl-CoA and carnitine esters were measured from freeze-clamped, non-working perfused hearts. In the presence of glucose, malonyl-CoA content was measured at 4.6μM. Insulin acutely raised this level by 34%. With insulin present, both palmitate (0.5mM) and adrenaline (5μM) decreased the malonyl-CoA content by 60% and 46% respectively. Insulin had no significant effect on the size of the combined pool of long-chain fatty acyl CoA and carnitine esters. Perfusion with palmitate significantly increased the content of both long-chain esters but decreased the ratio of acylcarnitine/acyl-CoA. Perfusion with adrenaline also significantly increased the content of both long-chain esters, in particular acylcarnitine. Adrenaline increased the content of the total long-chain ester pool by at least as much as palmitate.

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