Magnusson I, Schumann W C, Bartsch G E, Chandramouli V, Kumaran K, Wahren J, Landau B R
Department of Clinical Physiology, Karolinska Institute, Huddinge Hospital, Sweden.
J Biol Chem. 1991 Apr 15;266(11):6975-84.
To quantify intrahepatic Krebs cycle metabolism, phenyl acetate, excreted in urine as a glutamine conjugate, was given to healthy subjects infused with [3-14C]lactate. They were studied after 60 h of fasting and when given glucose after an overnight fast. Distributions of 14C in glutamate from urinary phenylacetylglutamine and blood glucose were determined. Corrections to the distributions because of the fixation of 14CO2 formed from the [3-14C]lactate were determined by administering [14C]bicarbonate. Comparisons of distributions in glucose and glutamate support the assumption that the glutamate distributions reflect those in hepatic alpha-ketoglutarate. From the distributions in glutamate, the extent of exchange of labeled with unlabeled carbons and relative flow rates in the cycle in liver were estimated. Dilution of 14C by 12C in the cycle was found in the fasted but not the fed state. In the fasted state, pyruvate carboxylation was estimated to be at least twice the rate of Krebs cycle flux and the rate of pyruvate's decarboxylation less than 1/25 the rate of its carboxylation. In the fed state, the rate of decarboxylation was estimated to be between one-sixth and one-half the rate of carboxylation. The rate of conversion of oxalacetate to fumarate in both states appeared to be greater than 6 times the rate of Krebs cycle flux.
为了量化肝内三羧酸循环代谢，将作为谷氨酰胺共轭物经尿液排出的苯乙酸给予输注了[3-¹⁴C]乳酸的健康受试者。在禁食60小时后以及禁食过夜后给予葡萄糖时对他们进行研究。测定了尿中苯乙酰谷氨酰胺和血糖中¹⁴C在谷氨酸中的分布。通过给予[¹⁴C]碳酸氢盐来确定因[3-¹⁴C]乳酸生成的¹⁴CO₂固定而对分布进行的校正。葡萄糖和谷氨酸中分布的比较支持这样的假设，即谷氨酸的分布反映了肝内α-酮戊二酸的分布。根据谷氨酸中的分布，估计了标记碳与未标记碳的交换程度以及肝脏中循环的相对流速。发现在禁食状态而非进食状态下，循环中¹⁴C被¹²C稀释。在禁食状态下，丙酮酸羧化的速率估计至少是三羧酸循环通量速率的两倍，而丙酮酸脱羧的速率不到其羧化速率的1/25。在进食状态下，脱羧速率估计在羧化速率的六分之一到二分之一之间。在两种状态下，草酰乙酸转化为富马酸的速率似乎都大于三羧酸循环通量速率的6倍。
