The influence of metal ions on malic enzyme activity and lipid synthesis in Aspergillus niger

In the presence of copper significant induction of citric acid overflow was observed, while concomitantly lower levels of total lipids were detected in the cells. Its effect was more obvious in a medium with magnesium as sole divalent metal ions, while in a medium with magnesium and manganese the addition of copper had a less pronounced effect. Since the malic enzyme was recognised as a supplier of reducing power in the form of reduced nicotinamide adenine dinucleotide phosphate for lipid biosynthesis, its kinetic parameters with regard to different concentrations of metal ions were investigated. Some inhibition was found with Fe2+ and Zn2+, while Cu2+ ions in a concentration of 0.1 mM completely abolished malic enzyme activity. The same metal ions proportionally reduced the levels of total lipids in Aspergillus niger cells. A strong competitive inhibition of the enzyme by Cu2+ was observed. It seemed that copper competes with Mg2+ and Mn2+ for the same binding site on the protein. (C) 2002 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.