Schalinske K L, Steele R D
Department of Nutritional Sciences, University of Wisconsin, Madison 53706.
Arch Biochem Biophys. 1989 May 15;271(1):49-55. doi: 10.1016/0003-9861(89)90254-3.
The quantitation of carbon flow through the folate-dependent one-carbon pool in regard to the synthesis of methionine from the amino acid precursor, serine, was determined in rat liver. Utilizing duodenal cannulated rats and in vivo tracer kinetic techniques where [3-14C]serine was continuously infused at a rate of 3.32 microCi/h, a steady-state (plateau) specific radioactivity was achieved within 200 min from the onset of the infusion period. This resulted in an irreversible loss rate of 431 +/- 12 mumol/h for hepatic serine. In conjunction with the specific radioactivity measurements of hepatic methionine, the percentage of the total entry into the hepatic methionine methyl carbon pool that came from serine (i.e., transfer quotient) was calculated to be 51.7 +/- 5.2%. Similar experiments utilizing [methyl-3H]methionine as the infusate resulted in a value of 112 +/- 6 mumol/h for the irreversible loss rate of hepatic methionine. Using the irreversible loss rate of methionine and the transfer quotient to methionine from serine, the flux of the beta-carbon of serine to remethylate homocysteine and generate methionine was calculated to be 57.9 mumol/h. These results not only present a methodology for the determination of folate-dependent carbon flow in vivo, but also demonstrate the high degree to which the homocysteine moiety of methionine is conserved in vivo to meet the methylation requirements in the rat.
在大鼠肝脏中，测定了从氨基酸前体丝氨酸合成蛋氨酸过程中，通过叶酸依赖性一碳池的碳流量。利用十二指肠插管大鼠和体内示踪动力学技术，以3.32微居里/小时的速率持续输注[3-14C]丝氨酸，在输注开始后的200分钟内达到了稳态（平台期）比放射性。这导致肝脏丝氨酸的不可逆损失率为431±12微摩尔/小时。结合肝脏蛋氨酸的比放射性测量，计算得出进入肝脏蛋氨酸甲基碳池的总碳中来自丝氨酸的百分比（即转移商）为51.7±5.2%。使用[甲基-3H]蛋氨酸作为输注液进行的类似实验得出，肝脏蛋氨酸的不可逆损失率为112±6微摩尔/小时。利用蛋氨酸的不可逆损失率和丝氨酸向蛋氨酸的转移商，计算得出丝氨酸的β-碳用于同型半胱氨酸再甲基化并生成蛋氨酸的通量为57.9微摩尔/小时。这些结果不仅提供了一种在体内测定叶酸依赖性碳流量的方法，还证明了蛋氨酸的同型半胱氨酸部分在体内高度保守，以满足大鼠的甲基化需求。
