A new mathematical approach to the metabolism of [14C]glucose, with applications to sensory ganglia of chicken embryos.

Abstract

Abstract— The available models of carbohydrate metabolism are not suitable for analysis of experiments on dorsal root ganglia of chicken embryos because they assume that certain products of the pentose cycle mix freely with those of glycolysis, which appears not to be true in this tissue, and because full isotopic equilibration, needed before the start of measurements, is not achieved while the excised ganglia are reasonably fresh. Therefore, new equations were developed which assume only a steady state of relevant metabolic intermediates and make use of the process of isotopic equilibration as a source of information. It is also assumed that an initially unknown but calculable fraction of the products of each pentose cycle re‐enters the next cycle, the remainder leaking either to glycolysis or to the incubation medium. From measurements of the time course of output of labelled CO2 in the presence of [1‐14C]‐ and [6‐14C]glucose and the incorporation and release in lactate of labelled C from [l‐14C]glucose, the equations permit the estimation of many features of carbohydrate metabolism, such as the partitioning of material between the pentose cycle and glycolysis, the partitioning of CO2 output between the pentose and citric acid cycles, the partitioning of the products of glycolysis between CO2 and other destinations, such as lactate, and the degree of recycling from one pentose cycle into the next. In addition, the time course of labelled CO2 output from [2‐14C]glucose can be predicted; this, by comparison with the observed output, serves to support some variants of the basic model, while invalidating others.In dorsal root ganglia from 15‐day chicken embryos, the assumption of a metabolic steady state was supported by a constant output of labelled CO2 from [l‐14C]glucose for 15 or more hours, except for the initial period of isotopic equilibration. By use of the new equations, it is concluded that in these ganglia (a) recycling in the pentose cycle can be 100% efficient in some incubation conditions, but not in others, (b) more CO2 is released from the pentose cycle than from the citric acid cycle, (c) large, quantifiable differences exist between the utilization of the various carbon atoms of glucose, and (d) a pool of intermediates within the pentose cycle, with a time constant of about 1 h, explains a large delay observed in the output of C‐6 of glucose into CO2, which occurs with a time constant as long as 5 h under some conditions. Under conditions where recycling is complete in the pentose cycle, this cycle must operate in isolation from glycolysis, which would otherwise convert much of the output of the pentose cycle to lactate. This may explain the role of fructose‐1,6‐diphospha‐tase in the tissue, without recourse to the oft‐proposed, puzzling, and ATP‐degrading‘futile cycle’between fructose‐6‐P and fructose‐1,6‐diP.It is proposed that the new equations may be suitable for similar analyses on some, but not all, other tissues.