Abstract
Glucose formed from [5-T]fructose in rat hepatocytes contains about 10 to 30% tritium. This does not appear to be due to fructose metabolism via hexokinase since neither the initial presence of glucose, nor wide variations in the original fructose concentration, have much effect on the relative labeling of glucose versus water from [5-T]fructose. Comparison of the T:14C ratios in glucose produced from [U-14C, 5-T]fructose and D-[U-14C, 2-T]glyceraldehyde indicate that there is tritium retention in the metabolism of fructose via the fructokinase-initiated pathway. The tritium retention can cause significant errors in the estimation of the futile cycle between fructose-1,6-P2 and fructose 6-P, by methods involving the use of [5-T]glucose.
Highlights
Glucose formed from [&T]fructose in rat hepatocytes contains about 10 to 30% of the tritium
Neglecting futile cycles or exchange reactions, all of the tritium on C-5 of fructose would remain in the glucose produced in this pathway
It might have been considered that glucose formed from [&T]fructose by the pathway outlined in Fig. 1 would have completely lost all the tritium. (As shown below, this is not correct.)
Summary
Glucose formed from [&T]fructose in rat hepatocytes contains about 10 to 30% of the tritium This does not appear to be due to fructose metabolism via hexokinase since neither the initial presence of glucose, nor wide variations in the original fructose concentration, have much effect on the relative labeling of glucose uersus water from [5-Tlfructose. Comparison of the T:“C ratios in glucose produced from[U-‘“C, 5-Tlfructose and D- [U-l%, 2-Tlglyceraldehyde indicate that there is tritium retention in the metabolism of fructose via the fructokinase-initiated pathway. The tritium retention can cause significant errors in the estimation of the futile cycle between fructose-1,6-P, and fructose 6-P, by methods involving the use of [5-Tlglucose. In studies on the estimation of futile cycles in the liver with [5-Tlglucose, it has been assumed that triose-P isomerase will cause complete exchange with water of the tritium on D- [2-Tlglyceraldehyde. We show in this paper that there is significant retention of tritium in the conversion of [5-Tlfructose to glucose in the fructokinaseinitiated pathway
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