Hoppel C L
Fed Proc. 1982 Oct;41(12):2853-7.
Carnitine is an essential factor in long-chain fatty acid oxidation. Carnitine acts as a carrier of fatty acyl groups from the cytoplasm to the mitochondrion. Long-chain acyl-CoA derivatives do not penetrate the mitochondrial inner membrane. Carnitine palmitoyltransferase A (CPT-A), located on the external surface of the inner membrane, catalyzes the conversion of cytoplasmic long-chain acyl-CoA and carnitine into acylcarnitine. The acylcarnitine is reconverted to intramitochondrial acyl-CoA by the action of carnitine palmitoyltransferase B located in the inner membrane. Now, the acyl-CoA is available for beta-oxidation in the matrix. An inner membrane carnitine-acylcarnitine translocase exchanges carnitine and acylcarnitine across the inner membrane but its role is long-chain acyl transfer has not been established. The tissue concentration of carnitine is important; liver carnitine is correlated with the rate of hepatic ketoacid production. In liver, malonyl-CoA, an intermediate in fatty acid synthesis, is proposed to regulate the activity of CPT-A. Studies using various purified transferases have not provided an answer to the question of whether the two activities expressed in mitochondria are separate enzymes. The absence of only CPT-A activity in isolated skeletal muscle mitochondria obtained from a patient with a lipid-storage myopathy suggests two separate activities.
肉碱是长链脂肪酸氧化的关键因素。肉碱作为脂肪酰基从细胞质到线粒体的载体。长链酰基辅酶A衍生物不能穿透线粒体内膜。位于内膜外表面的肉碱棕榈酰转移酶A（CPT-A）催化细胞质中的长链酰基辅酶A和肉碱转化为酰基肉碱。酰基肉碱通过位于内膜的肉碱棕榈酰转移酶B的作用重新转化为线粒体内的酰基辅酶A。现在，酰基辅酶A可用于基质中的β氧化。内膜肉碱-酰基肉碱转位酶在内膜上交换肉碱和酰基肉碱，但其在长链酰基转移中的作用尚未明确。肉碱的组织浓度很重要；肝脏中的肉碱与肝酮酸生成速率相关。在肝脏中，脂肪酸合成的中间产物丙二酰辅酶A被认为可调节CPT-A的活性。使用各种纯化转移酶的研究尚未回答线粒体中表达的两种活性是否为不同酶的问题。从一名脂质贮积性肌病患者分离出的骨骼肌线粒体中仅缺乏CPT-A活性，这表明存在两种不同的活性。
