Hexose monophosphate, acceleration

The synthesis of Oj makes possible acceleration of glucose metabolism in the hexose monophosphate shunt, yielding NADP+, which is then used for dehydrogenation of glucose 6-phosphate to 6-phosphogluconic aldehyde (K15). 

Indeed, concomitantly to the loss of enzyme activity, there is, as might be expected, a drop in ATP and other phosphorylated compounds. In addition to a loss of dehydrogenase and hexokinase, the concentration of NAD drops rapidly. It is again not known if this results from accelerated breakdown or from a reduced rate of biosynthesis. Those that postulate that the reduced NAD in red cells results from interference with its synthesis have proposed several mechanisms to explain the reduced rate of biosynthesis of NAD, including (1) a reduction in ribose-5-phos-phate resulting from the impaired activity of the hexose monophosphate shunt and(2) a decrease in the activity of the enzymes involved in the biosynthesis of the coenzymes. 

If NADPH is oxidized through the microsomal cytochrome system, then the pathway reducing NADP should be stimulated and the rate of glucose oxidation through the hexose monophosphate shunt should be accelerated in those tissues where the shunt operates. However, studies on the effect of thyroxine on glucose oxidation through the shunt pathways have yielded controversial results. 

In adipose tissues, insulin accelerates the dissimilation of glucose to CO2 through the Embden-Meyerhof pathway and the hexose monophosphate shunt and increases its utilization for glycogen and fatty acid synthesis. Insulin is without effect on fatty acid uptake and lipogenesis when glucose is absent from the medium. And the studies of Fain and Loken [132] have established that the antilipolytic effect of insulin is blocked by trypsin. Trypsin does not affect its inhibition through other metabolic interferences. A protein factor, possibly a receptor, probably is needed for insulin s action on adipose tissue. 

---