Bremer J
Institute of Medical Biochemistry, University of Oslo, Norway.
J Clin Chem Clin Biochem. 1990 May;28(5):297-301.
In animal cells long chain fatty acids are transferred into the mitochondria for oxidation as acylcarnitines. Carnitine palmitoyltransferase I in the outer membrane, and carnitine translocase plus carnitine palmitoyltransferase II in the inner membrane catalyse the transfer. Carnitine palmitoyltransferase I is inhibited by malonyl-CoA, an intermediate in fatty acid synthesis. In the liver of fasted, diabetic, or thyreotoxic animals this enzyme shows increased activity and less inhibition by malonyl-CoA. Peroxisomes also contain carnitine acyltransferases and a beta-oxidation enzyme system. This system is particularly active in the shortening of very long chain fatty acids. The carnitine acyltransferases of the peroxisomes presumably are active in the transfer of the shortened acyl-CoAs and the acetyl-CoA to the mitochondria for complete oxidation. The carnitine acyltransferases of the mitochondria can catalyse the formation of propionylcarnitine and branched chain acylcarnitines from branched chain amino acids, and methylthiopropionylcarnitine from methionine. Their formation may represent a "security valve" preventing acyl-CoA accumulation in the mitochondria. The liver, which normally releases carnitine for other tissues, releases the branched chain acylcarnitines even more easily. This may be important for the development of secondary carnitine deficiency in some inborn errors of metabolism which are accompanied by the accumulation of acyl-CoAs in the tissue.
在动物细胞中，长链脂肪酸以酰基肉碱的形式被转运到线粒体中进行氧化。外膜中的肉碱棕榈酰转移酶I以及内膜中的肉碱转位酶和肉碱棕榈酰转移酶II催化这种转运。肉碱棕榈酰转移酶I受到脂肪酸合成中间体丙二酰辅酶A的抑制。在禁食、糖尿病或甲状腺毒症动物的肝脏中，这种酶的活性增加，且受丙二酰辅酶A的抑制作用减弱。过氧化物酶体也含有肉碱酰基转移酶和β-氧化酶系统。该系统在极长链脂肪酸的缩短过程中特别活跃。过氧化物酶体的肉碱酰基转移酶可能在将缩短的酰基辅酶A和乙酰辅酶A转运到线粒体中进行完全氧化方面发挥作用。线粒体的肉碱酰基转移酶可以催化从支链氨基酸形成丙酰肉碱和支链酰基肉碱，以及从蛋氨酸形成甲基硫代丙酰肉碱。它们的形成可能代表一种“安全阀”，防止酰基辅酶A在线粒体中积累。通常向其他组织释放肉碱的肝脏，更容易释放支链酰基肉碱。这对于某些伴有组织中酰基辅酶A积累的先天性代谢缺陷中继发性肉碱缺乏的发展可能很重要。
