Wagenmakers A J, Salden H J, Veerkamp J H
Int J Biochem. 1985;17(9):957-65. doi: 10.1016/0020-711x(85)90240-x.
After incubation of muscle preparations with [U-14C]branched-chain amino acids or 2-oxo acids, radioactive metabolites were separated, identified and quantified. Homogenates of rat heart and skeletal muscle incubated with 4-methyl-2-oxopentanoate accumulated isovalerate, 3-hydroxyisovalerate and the corresponding carnitine esters. Incubation with 3-methyl-2-oxobutanoate resulted in the production of isobutyrate, 3-hydroxyisobutyrate and their carnitine esters. Addition of L-carnitine increased the production of the esters. The enzymes 3-methylcrotonyl-CoA carboxylase and 3-hydroxyisobutyric acid dehydrogenase apparently are inactive during incubation of muscle homogenates. With liver homogenates the degradation of both 2-oxo acids was more complete. Rat hemidiaphragms incubated with leucine, valine and isoleucine accumulated the corresponding branched-chain 2-oxo acids, fatty acids and hydroxylated fatty acids. The degradation of valine was markedly limited by the release of these metabolites. Considerable amounts (relatively smaller for valine) of radioactivity were also recovered in CO2 and glutamine and glutamate. Incubations with branched-chain 2-oxo acids gave the same radioactive products, except for glutamine and glutamate. Radioactivity was never found in lactate, pyruvate or alanine. These data indicate that the carbon-chains of amino acids entering the citric acid cycle in muscle, are not used for oxidation or for alanine synthesis, but are converted exclusively to glutamine.
在用[U-14C]支链氨基酸或2-氧代酸孵育肌肉制剂后，对放射性代谢物进行分离、鉴定和定量。用4-甲基-2-氧代戊酸孵育的大鼠心脏和骨骼肌匀浆积累了异戊酸、3-羟基异戊酸及其相应的肉碱酯。用3-甲基-2-氧代丁酸孵育导致产生异丁酸、3-羟基异丁酸及其肉碱酯。添加L-肉碱增加了酯的产生。在肌肉匀浆孵育过程中，3-甲基巴豆酰辅酶A羧化酶和3-羟基异丁酸脱氢酶显然没有活性。对于肝脏匀浆，两种2-氧代酸的降解更完全。用亮氨酸、缬氨酸和异亮氨酸孵育的大鼠半膈肌积累了相应的支链2-氧代酸、脂肪酸和羟基化脂肪酸。这些代谢物的释放显著限制了缬氨酸的降解。在二氧化碳、谷氨酰胺和谷氨酸中也回收了相当数量（缬氨酸相对较少）的放射性物质。用支链2-氧代酸孵育得到相同的放射性产物，但谷氨酰胺和谷氨酸除外。在乳酸、丙酮酸或丙氨酸中从未发现放射性。这些数据表明，进入肌肉柠檬酸循环的氨基酸碳链，不是用于氧化或丙氨酸合成，而是仅转化为谷氨酰胺。
