Inhibitory effect of 5-phosphoribosyl 1-pyrophosphate and ADP on the nonoxidative pentose phosphate pathway activity

The rate of the conversion of ribose 5-phosphate to hexose 6-phosphates by reaction of the non-oxidative pentose phosphate pathway was measured in the presence of various biological materials. Of 22 compounds tested, PRPP and ADP markedly inhibited the formation of hexose 6-phosphates from ribose 5-phosphate. The transketolase activity in beef liver enzyme preparation was extremely inhibited by PRPP and ADP, but the transaldolase activity was not inhibited. The mode of inhibition of transketolase by PRPP and ADP was a competitive one. The Ki value for PRPP was 0.14 mM and that for ADP 0.54 mM with respect to transketolase. We discuss the possible regulatory roles of ADP and PRPP on pentose phosphate metabolism in the pentose phosphate pathway.