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

[摘要]

Fatty acid-biosynthetic activity in rat liver cytosol fractions is much greater when the bivalent cation in the assay system is Mn²⁺ than when it is Mg²⁺. 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 Mg²⁺ and Mn²⁺ were established: (1) Mn²⁺ promotes acetyl-CoA carboxylase activity of the protomeric form of the enzyme under conditions in which Mg²⁺ does not; (2) Mn²⁺+ATP have little inhibitory effect on the polymerization of acetyl-CoA carboxylase whereas Mg²⁺+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 Mn²⁺ than with Mg²⁺. The role that each of these specific differences between Mn²⁺ and Mg²⁺ might play in causing liver cytosol preparations to have greater fatty acid-biosynthetic activity in the presence of Mn²⁺ is discussed.