Citrate lipogenesis

Both dehydrogenases of the pentose phosphate pathway can be classified as adaptive enzymes, since they increase in activity in the well-fed animal and when insulin is given to a diabetic animal. Activity is low in diabetes or starvation. Malic enzyme and ATP-citrate lyase behave similarly, indicating that these two enzymes are involved in lipogenesis rather than gluconeogenesis (Chapter 21). 

Lipogenesis is regulated at the acetyl-CoA carboxylase step by allosteric modifiers, phosphorylation/de-phosphorylation, and induction and repression of enzyme synthesis. Citrate activates the enzyme, and long-chain acyl-CoA inhibits its activity. Insulin activates acetyl-CoA carboxylase whereas glucagon and epinephrine have opposite actions. 

The biological effect of (-)-HCA stems from the inhibition of extramitochondrial cleavage of citrate to oxaloacetate and acetyl-CoA catalysed by ATPicitrate lyase. This limits the availability of acetyl-CoA units required for fatty acid synthesis and lipogenesis (Sullivan, 1984). The inhibition of ATP cit-rate lyase by (-)-HCA leads to less dietary carbohydrate utilization for the synthesis of fatty acids, resulting in more glycogen storage in the liver and muscles. Many in vitro... 

Figure 2.4. The provision of acetyl-CoA and NADPH for lipogenesis. PPP, pentose phosphate pathway T, tricarboxylate transporter K, a-ketoglutarate transporter. In ruminants, pyruvate dehydrogenase, ATP-citrate lyase and malic enzyme activities are low and perhaps non-functional. (From Murray et al., 1988. Harper s Biochemistry, 21st edn, p. 207, Appleton and Lange, Norwalk, CT reproduced with permission of The McGraw-Hill Companies).

Incubation of hepatocytes with glucagon also increases the phosphorylation of another lipogenic enzyme ATP-citrate lyase [127,128], The purified enzyme is also phosphorylated by cAMP-dependent protein kinase in vitro [128,129] resulting in a 2-fold increase in the Km for ATP [130], However, it is unclear what role this plays in the inhibition of lipogenesis. 

Pyruvate carboxylase is also important in lipogenesis. Citrate is transported out of mitochondria and cleaved in the cytosol to provide acetyl CoA for fatty acid synthesis the resultant oxaloacetate is reduced to malate, which undergoes oxidative decarboxylation to pyruvate, a reaction that provides at least half of the NADPH required for fatty acid synthesis. Pyruvate reenters the mitochondria and is carboxylated to oxaloacetate to maintain the process. 

Although the effects of insulin on postprandial metabolism are profound, other factors (e.g., substrate supply and allosteric effectors) also affect the rate and degree to which these processes occur. For example, elevated levels of fatty acids in blood promote lipogenesis in adipose tissue. Regulation by several allosteric effectors further ensures that competing pathways do not occur simultaneously for example, in many cell types fatty acid synthesis is promoted by citrate (an activator of acetyl-CoA carboxylase), whereas fatty acid oxidation is depressed by malonyl-CoA (an inhibitor of carnitine acyltransferase I activity). The control of fatty acid metabolism is described in Section 12.1. 

Acetyl CoA is converted to malonyl CoA and into fatty acids as described previously. The enzyme that carries out the first committed step for fatty-acid synthesis, acetyl CoA carboxylase, is finely controlled both allosterically and covalently. This enzyme can occur in a monomeric inactive form or a polymeric active form. One factor that affects this is citrate, which stimulates the polymeric or active form of acetyl CoA carboxylase. Thus, citrate plays an important role in lipogenesis as (1) a source of cytosolic acetyl CoA, (2) an allosteric positive effector of acetyl CoA carboxylase, and (3) a provider of oxaloacetate in the cytosol, which can allow transhydrogenation from NADH to NADPH. An allosteric inhibitor of acetyl CoA carboxylase that causes dissociation to the monomeric form is fatty-acyl CoA. Thus, if exogenous fatty acids are available, there is little reason to synthesize more fatty acids. Fatty-acyl CoA in the cytosol decreases malonyl CoA formation by inhibiting acetyl CoA carboxylase. 

In liver, the enzymes of the oxidative pathway exert, in comparison with adipose tissue, lower activity (Weber, 1963) consequently the malate, and also the citrate, pathways become more important in supplying hydrogen for lipogenesis. Malate difihises easily out of the mitochondria, therefore, favoring NADH transfer from mitochondria to cytoplasm and providing NADPH via the malic enzyme (Krebs et al., 1967). 

Wieland (1961) proposed a theory which attributes the defect in lipogenesis to an altered NADH NAD ratio in diabetes. The increased rate of fatty acid oxidation leads to a raised NADH NAD within the diabetic liver cell. This causes the NADH-dependent ox-aloacetate-malate equilibrium to shift in favor of malate and, therefore, a lowering of the limiting component of the citric acid cycle, viz. oxaloacetate. This would result in extreme intracellular deficiency of citrate. While this theory might have some pertinence for the liver, the same changes apparently do not take place in heart. The citrate concentration is elevated in hearts from diabetic or fasted rats (Parmeggiani and Bowman, 1963 Garland and Randle, 1964). 

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