The regulation of mitochondrial-bound hexokinases in the liver

A functional coupling between bound hexokinase and the inner mitochondrial compartment has been shown. It is based structurally on the binding of hexokinase to a pore protein which is present in zones of contact between the two boundary membranes. The latter was observed by electron microscopic localization of antiporin and hexokinase at the mitochondrial surface. The four isoenzymes present in liver differ considerably in their activity after binding to the mitochondrial surface. This was found by binding studies using the four isoenzymes isolated from the supernatant. Isoenzyme IV did not bind at all. Isoenzymes I-III did bind and became activated: I, 5.9-fold; II, 39-fold; and III, 1.3-fold. These results suggest that the in vivo activity of hexokinase in the mitochondrial fraction is much larger than so far observed. Furthermore the binding of isoenzymes was differently affected by metabolites. Glucose-6-phosphate exclusively desorbed isoenzyme I from the mitochondrial membrane whereas free fatty acids predominantly liberated isoenzymes II and III. A reciprocal change of the levels of free fatty acids and glucose 6-phosphate in livers of starved rats therefore, can explain why exclusively mitochondrial-bound isoenzymes II and III decreased 10-fold while at the same time isoenzyme I increased.