Conformational change of the catalytic subunit of glucose-6-phosphatase inrat liver during the fetal-to-neonatal transition

The glucose-6-phosphatase system was investigated in fetal rat liver micros omal vesicles. Several observations indicate that the orientation of the ca talytic subunit is different in the fetal liver in comparison with the adul t form: (i) the phosphohydrolase activity was not latent using glucose-6-ph osphate as substrate, and in the case of other phosphoesters it was less la tent; (ii) the intravesicular accumulation of glucose upon glucose-6-phosph ate hydrolysis was lower; (iii) the size of the intravesicular glucose-6-ph osphate pool was independent of the glucose-6-phosphatase activities; (iv) antibody against the loop containing the proposed catalytic site of the enz yme inhibited the phosphohydrolase activity in fetal but not in adult rat l iver microsomes. Glucose-6-phosphate, phosphate, and glucose uptake could b e detected by both light scattering and/or rapid filtration method in fetal liver microsomes; however, the intravesicular glucose-6-phosphate and gluc ose accessible spaces were proportionally smaller than in adult rat liver m icrosomes. These data demonstrate that the components of the glucose-6-phos phatase system are already present, although to a lower extent, in fetal li ver, but they are functionally uncoupled by the extravesicular orientation of the catalytic subunit.