Use of [2-14C]glucose and [5-14C]glucose for evaluating the mechanism and quantitative significance of the 'liver-cell' pentose cycle.
1. 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 glucose 6-phosphate. The second procedure involves the metabolism of [2-14C]glucose and the isolation and degradation of a triose phosphate derivative, usually lactate or glycerol. 2. Measurements of 14C in C-2 and C-5 of glucose 6-phosphate 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. 3. 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. 4. The effect of transaldolase and transketolase 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 glucose 6-phosphate, which could interfere with a clear interpretation of the data using [2-14C]- and [5-14C]glucose are discussed.
D002250:Carbon Radioisotopes; D004099:Dihydroxyacetone Phosphate / Q000378:metabolism; D005943:Gluconeogenesis; D005947:Glucose / Q000378:metabolism*; D005986:Glyceraldehyde 3-Phosphate / Q000378:metabolism; D008954:Models, Biological; D010428:Pentosephosphates / Q000378:metabolism*; D014153:Transaldolase / Q000378:metabolism
D002250:Carbon Radioisotopes; D010428:Pentosephosphates; D005986:Glyceraldehyde 3-Phosphate; D004099:Dihydroxyacetone Phosphate; D014153:Transaldolase; D005947:Glucose