Use of [2-14C]glucose and [5-14C]glucose for evaluating the mechanism and quantitative significance of the ‘liver-cell’ pentose cycle

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Abstract
Expressions are derived for the steady-state measurement of the quantitative contribution of the liver-type pentose phosphate cycle to glucose metabolism by tissues. One method requires the metabolism of [5-14C]glucose followed by the isolation and degradation of G-6-P. The 2nd procedure involves the metabolism of [2-14C]glucose and the isolation and degradation of a triose phosphate derivative, usually lactate or glycerol. Measurements of 14C in C-2 and C-5 of G-6-P are required and the values of the C-2/C-5 ratios can be used to calculate the quantitative contribution of the L-type pentose cycle in all tissues. The measurement of 14C in C-1, C-2 and C-3 of triose phosphate derivatives can be used to calculate the quantitative contribution of the L-type pentose cycle relative to glycolysis. The effect of transaldolase (EC 2.2.1.2) and transketolase (EC 2.2.1.1) exchange reactions, reactions of gluconeogenesis and non-oxidative formation of pentose 5-phosphate, isotopic equilibration of triose phosphate pools and isotopic equilibration of fructose 6-phosphate and G-6-P, which could interfere with a clear interpretation of the data using [2-14C]- and [5-14C]glucose, are discussed.