Landau B R
Department of Medicine, Case Western Reserve University School of Medicine, Cleveland, OH.
Metabolism. 1993 Apr;42(4):457-62. doi: 10.1016/0026-0495(93)90103-u.
Approaches measuring futile cycling of glucose and fructose-6-phosphate (fructose-6-P) in liver in vivo depend on assumptions about the fates of hydrogens bound to specific carbons of glucose. Thus, 3H of [2-3H]glucose has been assumed to be completely removed after its conversion to glucose-6-P, [3-3H]glucose after its conversion to fructose-1,6-bisP, and [6-3H]glucose not at all. Previous measurements have shown that these assumptions are incompletely fulfilled. Corrections to estimates of cycling can be made when detritiations of [2-3H]glucose and [3-3H]glucose are not complete, and detritiation of [6-3H]glucose occurs. How the corrections can be made is presented using data previously reported on giving labeled glucoses to humans after an overnight fast and on infusing a glucose load. Estimates of glucose cycling nearly double, and that of fructose-6-P cycling almost triples. Estimates of hepatic glucose production as measured with [6-3H]glucose decrease. Correction of estimates of cycling under other conditions may very well be similarly affected. Thus, rates of glucose and fructose-6-P cycling appear to be substantially more than previously estimated. Quantitation under a given condition requires measurements to be made of the extent to which assumptions as to the fate of labeled hydrogen of the glucoses are fulfilled. The uncertain extent of exchange of label catalyzed by transaldolase and detritiation in the pentose cycle, the failure of fructose-6-P cycling to be expressed through detritiation of 3H from [3-3H]glucose, and possible isotope effects still limit the confidence that can be placed in such estimates.
体内测量肝脏中葡萄糖和6-磷酸果糖（果糖-6-P）无效循环的方法依赖于对与葡萄糖特定碳原子结合的氢的去向的假设。因此，[2-³H]葡萄糖的³H在转化为6-磷酸葡萄糖后被假定完全去除，[3-³H]葡萄糖在转化为1,6-二磷酸果糖后被假定完全去除，而[6-³H]葡萄糖则被假定完全没有去除。先前的测量表明，这些假设并未完全得到满足。当[2-³H]葡萄糖和[3-³H]葡萄糖的去氚化不完全且[6-³H]葡萄糖发生去氚化时，可以对循环估计值进行校正。利用先前报道的有关在禁食过夜后给人类输注标记葡萄糖以及输注葡萄糖负荷的数据，介绍了如何进行校正。葡萄糖循环的估计值几乎翻倍，而6-磷酸果糖循环的估计值几乎增至三倍。用[6-³H]葡萄糖测量的肝脏葡萄糖生成估计值降低。在其他条件下对循环估计值的校正很可能也会受到类似影响。因此，葡萄糖和6-磷酸果糖循环的速率似乎比先前估计的要高得多。在给定条件下进行定量需要测量葡萄糖标记氢去向假设的满足程度。转醛醇酶催化的标记交换程度以及戊糖循环中的去氚化程度不确定，6-磷酸果糖循环未能通过[3-³H]葡萄糖的³H去氚化来体现，以及可能存在的同位素效应，仍然限制了对此类估计的置信度。
