Katsurada A, Fukuda H, Iritani N
Biochim Biophys Acta. 1986 Sep 12;878(2):200-8.
When fasted rats were refed for 4 days with a carbohydrate and protein diet, a carbohydrate diet (without protein) or a protein diet (without carbohydrate), the effects of dietary nutrients on the fatty acid synthesis from injected tritiated water, the substrate and effector levels of lipogenic enzymes and the enzyme activities were compared in the livers. In the carbohydrate diet group, although acetyl-CoA carboxylase was much induced and citrate was much increased, the activity of acetyl-CoA carboxylase extracted with phosphatase inhibitor and activated with 0.5 mM citrate was low in comparison to the carbohydrate and protein diet group. The physiological activity of acetyl-CoA carboxylase seems to be low. In the protein diet group, the concentrations of glucose 6-phosphate, acetyl-CoA and malonyl-CoA were markedly higher than in the carbohydrate and protein group, whereas the concentrations of oxaloacetate and citrate were lower. The levels of hepatic cAMP and plasma glucagon were high. The activities of acetyl-CoA carboxylase and also fatty acid synthetase were low in the protein group. By feeding fat, the citrate level was not decreased as much as the lipogenic enzyme inductions. Comparing the substrate and effector levels with the Km and Ka values, the activities of acetyl-CoA carboxylase and fatty acid synthetase could be limited by the levels. The fatty acid synthesis from tritiated water corresponded more closely to the acetyl-CoA carboxylase activity (activated 0.5 mM citrate) than to other lipogenic enzyme activities. On the other hand, neither the activities of glucose-6-phosphate dehydrogenase and malic enzyme (even though markedly lowered by diet) nor the levels of their substrates appeared to limit fatty acid synthesis of any of the dietary groups. Thus, it is suggested that under the dietary nutrient manipulation, acetyl-CoA carboxylase activity would be the first candidate of the rate-limiting factor for fatty acid synthesis with the regulations of the enzyme quantity, the substrate and effector levels and the enzyme modification.
给禁食的大鼠分别用碳水化合物和蛋白质饮食、碳水化合物饮食（不含蛋白质）或蛋白质饮食（不含碳水化合物）再喂养4天，比较了不同饮食营养成分对肝脏中由注射的氚水合成脂肪酸、生脂酶的底物和效应物水平以及酶活性的影响。在碳水化合物饮食组中，尽管乙酰辅酶A羧化酶被大量诱导且柠檬酸大量增加，但用磷酸酶抑制剂提取并用0.5 mM柠檬酸激活后的乙酰辅酶A羧化酶活性，与碳水化合物和蛋白质饮食组相比仍较低。乙酰辅酶A羧化酶的生理活性似乎较低。在蛋白质饮食组中，6-磷酸葡萄糖、乙酰辅酶A和丙二酰辅酶A的浓度明显高于碳水化合物和蛋白质组，而草酰乙酸和柠檬酸的浓度较低。肝脏中cAMP和血浆胰高血糖素水平较高。蛋白质组中乙酰辅酶A羧化酶和脂肪酸合成酶的活性较低。喂食脂肪后，柠檬酸水平的降低幅度不如生脂酶诱导的降低幅度大。将底物和效应物水平与Km和Ka值进行比较，乙酰辅酶A羧化酶和脂肪酸合成酶的活性可能受这些水平的限制。由氚水合成脂肪酸的过程与乙酰辅酶A羧化酶活性（用0.5 mM柠檬酸激活）的相关性，比与其他生脂酶活性的相关性更紧密。另一方面，6-磷酸葡萄糖脱氢酶和苹果酸酶的活性（即使因饮食而显著降低）及其底物水平，似乎均未限制任何饮食组的脂肪酸合成。因此，有人提出，在饮食营养成分的调控下，乙酰辅酶A羧化酶活性可能是脂肪酸合成限速因子的首要候选者，这一过程涉及酶量、底物和效应物水平以及酶修饰的调节。
