Kim Jong-Yeon, Koves Timothy R, Yu Geng-Sheng, Gulick Tod, Cortright Ronald N, Dohm G Lynis, Muoio Deborah M
Department of Biochemistry, East Carolina University, Greenville, North Carolina 27858, USA.
Am J Physiol Endocrinol Metab. 2002 May;282(5):E1014-22. doi: 10.1152/ajpendo.00233.2001.
Carnitine palmitoyltransferase I (CPT I), which is expressed as two distinct isoforms in liver (alpha) and muscle (beta), catalyzes the rate-limiting step in the transport of fatty acid into the mitochondria. Malonyl-CoA, a potent inhibitor of CPT I, is considered a key regulator of fatty acid oxidation in both tissues. Still unanswered is how muscle beta-oxidation proceeds despite malonyl-CoA concentrations that exceed the IC(50) for CPT Ibeta. We evaluated malonyl-CoA-suppressible [(14)C]palmitate oxidation and CPT I activity in homogenates of red (RG) and white (WG) gastrocnemius, soleus (SOL), and extensor digitorum longus (EDL) muscles. Adding 10 microM malonyl-CoA inhibited palmitate oxidation by 29, 39, 60, and 89% in RG, SOL, EDL, and WG, respectively. Thus malonyl-CoA resistance, which correlated strongly (0.678) with absolute oxidation rates (RG > SOL > EDL > WG), was greater in red than in white muscles. Similarly, malonyl-CoA-resistant palmitate oxidation and CPT I activity were greater in mitochondria from RG compared with WG. Ribonuclease protection assays were performed to evaluate whether our data might be explained by differential expression of CPT I splice variants. We detected the presence of two CPT Ibeta splice variants that were more abundant in red compared with white muscle, but the relative expression of the two mRNA species was unrelated to malonyl-CoA resistance. These results provide evidence of a malonyl-CoA-insensitive CPT I activity in red muscle, suggesting fiber type-specific expression of distinct CPT I isoforms and/or posttranslational modulations that have yet to be elucidated.
肉碱棕榈酰转移酶I（CPT I）在肝脏（α型）和肌肉（β型）中以两种不同的同工型表达，催化脂肪酸转运到线粒体中的限速步骤。丙二酸单酰辅酶A是CPT I的强效抑制剂，被认为是这两种组织中脂肪酸氧化的关键调节因子。尽管丙二酸单酰辅酶A的浓度超过了CPT Iβ的半数抑制浓度（IC50），但肌肉中的β氧化如何进行仍未得到解答。我们评估了红色（RG）和白色（WG）腓肠肌、比目鱼肌（SOL）和趾长伸肌（EDL）匀浆中丙二酸单酰辅酶A抑制的[14C]棕榈酸氧化和CPT I活性。添加10μM丙二酸单酰辅酶A分别使RG、SOL、EDL和WG中的棕榈酸氧化抑制了29%、39%、60%和89%。因此，丙二酸单酰辅酶A抗性与绝对氧化速率密切相关（0.678）（RG>SOL>EDL>WG），在红色肌肉中比白色肌肉中更大。同样，与WG相比，RG线粒体中丙二酸单酰辅酶A抗性的棕榈酸氧化和CPT I活性更高。进行核糖核酸酶保护试验以评估我们的数据是否可以用CPT I剪接变体的差异表达来解释。我们检测到两种CPT Iβ剪接变体的存在，它们在红色肌肉中比白色肌肉中更丰富，但这两种mRNA种类的相对表达与丙二酸单酰辅酶A抗性无关。这些结果提供了红色肌肉中存在丙二酸单酰辅酶A不敏感的CPT I活性的证据，表明不同CPT I同工型的纤维类型特异性表达和/或尚未阐明的翻译后调节。
