Glycolysis-citric acid cycle interrelation: a new approach and some insights in cellular and subcellular...

In the development of an integrated approach to study metabolic compartmentation and regulation in brain, we have emphasized the importance, versatility, and need to exploit the recent methodological advances in (1) NMR spectroscopy, (2) primary cultures of neurons and glial cells, and (3) subcellular fractionation (especially brain mitochondrial isolation). The integrated approach has the advantage of being able to draw data and inferences based on some combination of results derived from in vivo, cellular, and subcellular studies. For example, some in vivo NMR data may suggest that an enzymatic step may be rate-limiting in a particular pathway. This information may be used to frame testable hypotheses and questions that can be investigated in experiments involving primary cultures of neural cells and subcellular fractions. Subsequently, the data from such in vitro studies could serve as the bases for constructing the hypothetical framework for predicting the regulatory role, in vivo, of the enzyme in the pathway. We have discussed the known as well as the as yet ill-defined facets of the cellular and subcellular aspects of the glycolysis-citric acid cycle interrelation and have attempted to illustrate how such an integrated approach could be applied to generate testable hypotheses for investigating the mechanisms concerned with metabolic compartmentation and regulation in brain. In the process of the illustration, we discuss some of the evidence in support of the general hypothesis that the transfer of reducing equivalents across the inner mitochondrial membrane plays a major role in mediating the coupling of the glycolytic flux to that of the citric acid cycle. We have given some indications as to how this hypothesis could be further investigated employing our approach. Moreover, we hope that other workers will find this integrated approach useful in designing multidisciplinary studies to investigate mechanistic issues related to this important theme.