Gluconeogenesis from Serine by the Serine‐Dehydratase‐Dependent Pathway in Rat Liver

Isolated perfused livers from rats made deficient in pyruvate carboxylase by feeding an avidin‐rich, biotin‐deficient diet produced much less glucose than those from the corresponding normal animals (0.23 ± 0.08 and 0.83 ± 0.05 μmol/g liver per min, respectively). The ability to form glucose from alanine was also markedly impaired, while that from fructose was not similarly affected. Parenteral administration of biotin led within 3 h to full recovery of the activity of hepatic pyruvate carboxylase, as well as to restoration of the ability to form glucose from serine, both by the isolated liver and by the whole animal (measured in the latter case by the formation of [14C]glucose from [14C]serine). In the perfused liver, gluconeogenesis from hydroxypyruvate and glycerate, the specific intermediates of the potential alternative pathway from serine to glucose mediated by serine pyruvate transaminase, was smaller than that from serine. The serine dehydratase in the liver of fasted rats is in terms of V (2.3 ± 0.7 U/g) and Km (approx. 40 mM) quantitatively suited to account for the rate of gluconeogenesis from serine in the perfused liver, both when offered as the only precursor at high concentration and when offered as part of a mixture of precursors at physiological concentrations. In contrast, the efficiency of the so‐called serine pyruvate transaminase to act on serine in physiological conditions appears to be utterly insufficient to permit a major contribution to gluconeogenesis from serine by this means. These results indicate that gluconeogenesis from serine in rat liver proceeds essentially by the serine‐dehydratase‐mediated pathway.