Glucose metabolism in the placenta

The placenta of the guinea pig has been found to possess hexokinase activity attributable to the low K_m, nonspecifie type, but no high K_m glucokinase or ketohexokinase activity could be demonstrated. The relative rates of glycolysis, determined with glucose and glucose 6-phosphate as substrates, indicated that the phosphorylation of glucose was a rate-limiting step. No change in the activity of several glycolytic enzymes was noted during gestation, but the activities of glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase decreased toward term. The nature of the phosphatase action in vitro upon glucose 6-phosphate and fructose 1,6-diphosphate as substrates suggested that it was due to that of a nonspecific phosphatase; no evidence could be found for the presence in the placentas of rats, guinea pigs, rabbits, monkeys, and humans at several gestational ages of specific phosphatase acting upon those substrates such as occur in liver and kidney tissue. The placenta thus does not contain several of the enzymes believed to be of importance in the regulation of blood glucose by the mature liver. If it has a role as a temporary fetal liver during gestation in this respect, then other control mechanisms presumably operate.