Abstract
Phosphoenolpyruvate carboxykinase (PEPCK) occurs in hepatic cytosol and mitochondria of most mammalian species. The cytosolic enzyme responds adaptively to physiological conditions that vary the rate of gluconeogenesis. It is inhibited in vivo by quinolinic acid, either administered directly or produced in the liver from administered tryptophan. Cytosolic PEPCK is maximally active with the nucleotide substrate complexed with Mg2+ and a trace amount of divalent transition metal salt (Mn2+, Fe2+, Co2+, Cd2+). The activity of the cytosol enzyme is increased 3 to 5 fold by incubation with Fe2+ prior to assaying for activity. Quinolinic acid inhibits purified, cytosolic PEPCK in the presence of Fe2+ but not in the presence of Mn2+ or in the absence of divalent transition metal salts. This indicates that Mn2+ is not the natural activator of cytosolic PEPCK and that Fe2+ may be.
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