The mechanism of fructose formation in goat placenta with special reference to the possible involvement of sorbitol or of phosphoric acid esters

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Abstract
Experiments designed to examine current hypotheses concerning the mechanism of fructose formation from glucose in ungulate placentas have been carried out. The goat placenta is permeable to sorbitol. Injection of sorbitol into the maternal circulation increases the concentration of both fructose and glucose in the maternal blood, but injection into the fetal circulation increases only the fetal-blood fructose. In the absence of the fetus, perfusion in situ of the fetal side of the goat placenta with sorbitol does not increase the rate of fructose formation. The presence or absence of sorbitol dehydrogenase (ketose reductase) in the placentas of 15 species of animals cannot be correlated with the nature of the principal fetal-blood sugar. Injection of glucose into the maternal blood leads to a marked increase in fructose 1.6-diphosphate in the placenta and to a decrease in placental inorganic orthophosphate. Current hypotheses of fructose formation from glucose in ungulate placenta are reviewed in the light of these findings.