The significance of sedoheptulose 1,7-bisphosphate in the metabolism and regulation of the pentose pathway in liver.

Rat liver cytosolic enzyme preparation catalyses the formation of sedoheptulose 1,7-P2 (60% of total heptulose-P formed) from hexose 6-P and triose 3-P (reverse mode of pentose pathway operation). Smaller amounts of sedoheptulose 1,7-P2 are also formed from ribose 5-P during the non-oxidative synthesis of hexose 6-P (forward pentose pathway operation). The apparent absence of erythrose 4-P in biological systems may be explained by its contribution to carbons 4,5,6 and 7 of sedoheptulose 1,7-P2 as well as its pronounced ability to exist in dimeric form. Apart from the aldolase catalyzed formation of sedoheptulose 1,7-P2, 6-phosphofructokinase also catalyses its formation from sedoheptulose 7-P and fructose 1,6-bisphosphatase catalyses its dephosphorylation. These three enzymes may contribute to the regulation of carbon flux through the near equilibrium reactions of the non-oxidative pentose phosphate pathway in vivo. The phosphotransferase enzyme of the L-type pentose pathway is also able to catalyse the interconversion of sedoheptulose mono and bisphosphates via D-glycero D-ido octulose-P.