Guder W G, Wagner S
Institut für Klinische Chemie, Städt, Krankenhaus München-Bogenhausen, FRG.
J Clin Chem Clin Biochem. 1990 May;28(5):347-50.
The kidney plays a major role in carnitine biosynthesis, excretion and acylation. Unlike in the rat, human kidney contains all enzymes needed to form carnitine from trimethyllysine in activities exceeding those of the liver. This carnitine precursor is found to be increased in plasma of patients with chronic renal failure. Free carnitine formed in the kidney as well as carnitine reabsorbed from the glomerular filtrate may be acylated in the proximal tubule. Isolated rat cortical tubule suspensions contain total carnitine concentrations of 2.85 mumols/g protein. During incubation over 60 min the acylcarnitine/carnitine ratio decreased, indicating deacylation of acylcarnitine in proximal tubules. Exogenous carnitine was acylated at a rate of 35 mumols/h.g protein. Besides pyruvate and acetate, ketone bodies stimulated the acylation rate severalfold, indicating that these substrates are a major source of acetyl-CoA for the acylation reaction. This may explain the higher acetylcarnitine/carnitine ratio found in urine under ketotic conditions.
肾脏在肉碱的生物合成、排泄及酰化过程中发挥着主要作用。与大鼠不同，人类肾脏含有从三甲基赖氨酸合成肉碱所需的所有酶，其活性超过肝脏。已发现慢性肾衰竭患者血浆中的这种肉碱前体增加。在肾脏中形成的游离肉碱以及从肾小球滤液中重吸收的肉碱可能在近端小管中被酰化。分离的大鼠皮质小管悬浮液中总肉碱浓度为2.85微摩尔/克蛋白质。在60分钟的孵育过程中，酰基肉碱/肉碱比值下降，表明近端小管中酰基肉碱发生了去酰化。外源性肉碱的酰化速率为35微摩尔/小时·克蛋白质。除了丙酮酸和乙酸盐外，酮体可使酰化速率提高数倍，表明这些底物是酰化反应中乙酰辅酶A的主要来源。这可能解释了在酮症状态下尿液中发现的较高的乙酰肉碱/肉碱比值。
