The pentose phosphate cycle is regulated by NADPH/NADP ratio in rat liver.

The changes in the activity of the pentose phosphate cycle produced by the activation or inhibition of different NADPH-consuming pathways have been studied. The inhibition of fatty acid synthesis by kynurenate produced to the same extent, inhibition of the pentose phosphate cycle activity and an increase (about twofold) in the NADPH/NADP ratio. The addition of ter-butyl-hydroperoxide or paraquat, which is metabolized via NADPH-consuming pathways, produced the activation of the pentose phosphate cycle and a decrease in the NADPH/NADP ratio (about threefold). The plot of the NADPH/NADP ratio versus the pentose phosphate cycle activity gave a straight line with a regression index of 0.999. The regulation of the pentose phosphate cycle mainly by the intracellular NADPH/NADP ratio is discussed.