Differences between manganese and magnesium ions with regard to fatty acid biosynthesis, acetyl-coenzyme A carboxylase activity and malonyl-coenzyme A decarboxylation.

Fatty acid-biosynthetic activity in rat liver cytosol fractions is much greater when the bivalent cation in the assay system is Mn(2+) than when it is Mg(2+). This difference between bivalent cations can be abolished if the cytosol fractions are preincubated with isocitrate and the bivalent cation for 30min before assay of fatty acid-biosynthetic activity. In a search for the biochemical basis of this phenomenon, the following differences between Mg(2+) and Mn(2+) were established: (1) Mn(2+) promotes acetyl-CoA carboxylase activity of the protomeric form of the enzyme under conditions in which Mg(2+) does not; (2) Mn(2+)+ATP have little inhibitory effect on the polymerization of acetyl-CoA carboxylase whereas Mg(2+)+ATP are markedly inhibitory; (3) under conditions in which utilization of malonyl-CoA in condensation reactions is prevented, the steady-state concentration of malonyl-CoA formed by a cytosol fraction is much greater with Mn(2+) than with Mg(2+). The role that each of these specific differences between Mn(2+) and Mg(2+) might play in causing liver cytosol preparations to have greater fatty acid-biosynthetic activity in the presence of Mn(2+) is discussed.