Bismut H, Hers H G, Van Schaftingen E
Laboratoire de Chimie Physiologique, Université Catholique de Louvain, Brussels, Belgium.
Eur J Biochem. 1993 Apr 15;213(2):721-6. doi: 10.1111/j.1432-1033.1993.tb17812.x.
Gopher et al [Gopher, A., Vaisman, N., Mandel, H. & Lapidot, A. (1990) Proc. Natl Acad. Sci. USA 87, 5449-5453] recently reported that about 50% of the glucose formed from [U-13C]fructose infused nasogastrically in children contained 13C3 adjacent to 13C4. Assuming a high isotopic dilution of the triosephosphate pool, the authors concluded that about 50% of the fructose converted to glucose in liver and intestine bypassed the classical aldolase pathway, utilizing a hypothetical direct pathway that would involve the phosphorylation of fructose 1-phosphate to fructose 1,6-bisphosphate. The present work was undertaken in order to establish to what extent the conversion of fructose to glucose in the intestine could account for this unexpected isotopic distribution. The technique of everted sleeves was used to define the rate of conversion of [U-14C]glucose and [U-14C]fructose in the small intestine of 24-h-fasted rabbits. It appeared that, at the low concentration of fructose used by Gopher et al., almost as much fructose was converted to glucose as remained unmodified in the tissue. Fructose uptake was not inhibited by glucose, and the presence of all the necessary enzymes in the tissue indicated that the fructose to glucose conversion occurred by the aldolase pathway. Remarkably, this conversion operated with an isotopic dilution not exceeding 25%, due to the low rate of glucose metabolism and the near absence of gluconeogenesis from lactate. It can, therefore, be postulated that, in the presence of pure [U13C]fructose, the triosephosphate pool is highly enriched in 13C with little dilution by 12C, essentially giving rise to [U-13C]glucose, as reported by Gopher et al. There is, therefore, no need to postulate the participation of a direct pathway.
戈弗等人[戈弗，A.，维斯曼，N.，曼德尔，H. & 拉皮多特，A.（1990年）《美国国家科学院院刊》87卷，5449 - 5453页]最近报告称，经鼻胃管注入儿童体内的[U - 13C]果糖生成的葡萄糖中，约50%含有与13C4相邻的13C3。假设磷酸丙糖池存在高同位素稀释，作者得出结论，肝脏和肠道中约50%转化为葡萄糖的果糖绕过了经典的醛缩酶途径，利用了一种假设的直接途径，该途径涉及将1 - 磷酸果糖磷酸化为1,6 - 二磷酸果糖。开展本研究是为了确定肠道中果糖向葡萄糖的转化在多大程度上能够解释这种意外的同位素分布。采用外翻肠段技术来确定禁食24小时的兔子小肠中[U - 14C]葡萄糖和[U - 14C]果糖的转化速率。结果显示，在戈弗等人使用的低果糖浓度下，转化为葡萄糖的果糖量几乎与组织中未被修饰的果糖量相同。果糖摄取不受葡萄糖抑制，并且组织中存在所有必需的酶表明果糖向葡萄糖的转化是通过醛缩酶途径进行的。值得注意的是，由于葡萄糖代谢速率低以及几乎不存在由乳酸生成葡萄糖的糖异生作用，这种转化的同位素稀释不超过25%。因此，可以推测，在存在纯[U13C]果糖的情况下，磷酸丙糖池高度富集13C，几乎没有被12C稀释，基本上会产生[U - 13C]葡萄糖，正如戈弗等人所报道的那样。因此，无需假设存在直接途径的参与。
