The pentose phosphate pathway and NADPH utilization in rat liver

The role of the pentose phosphate pathway as a source of NADPH required for cytoplasmic processes such as lipogenesis and detoxification reactions has been examined. G6PDH and 6PGDH are known to be strongly inhibited by the high NADPH/NADP ratio which is thought to occur in the cytoplasm but no effector at physiological concentrations has yet been found which can overcome this inhibition. Initially a possible role for F2, 6P2 as an activator of G6PDH, 6PGDH and FAS was investigated but no significant effect of this regulatory metabolite on any of these enzymes was discovered. An attempt was also made to demonstrate the reported reversal of the inhibition by GSSG and the cofactor reported by Eggleston and Krebs (1974). This too could not be demonstrated. In the course of the work, a cytosolic NADPH-consuming reaction has been characterized. This has been shown to involve the reaction of a peptide-substrate with a cytoplasmic reductase specific for NADPH and a high affinity for the peptide. The physiological role of this reaction remains to be established, but it has been observed that the reaction exhibits a diurnal variation, the pattern of which is the reverse of that observed with lipogenesis. The low molecular weight peptide, which appears to be distinct from glutathione, contains cystine residues which are apparently reduced in the presence of NADPH, resulting in the appearance of free thiol groups. The peptide may be phosphorylated but the nature of the linkage between the peptide and phosphate has not been established. A possible role for this and other NADPH-dependent reactions in the regulation of the pentose phosphate pathway is discussed in this thesis.