alpha-Isopropylmalate synthase from yeast. A zinc metalloenzyme.

Highly purified alpha-isopropylmalate synthase (3-hydroxy-4-methyl-3-carboxyvalerate 2-oxo-3-methylbutyrate-lysase (coA-acetylating), EC 4.1.3.12) from Saccharomyces cerevisiae is inactivated by various chelating agents. Atomic absorption spectrometry indicates that the enzyme contains approx. four gatoms of zinc per dimer of molecular weight of 130 000. Dialysis against ethylenediaminetetraacetic acid at an initial concentration of 0.1 mM reduces the zinc content to about two gatoms of zinc per dimer. While such enzyme remains active, it has altered kinetic properties and is stimulated by Mn2+, in contrast to untreated enzyme. Dialysis against ethylenediaminetetraacetic acid at an initial concentration of 50 mM reduces the zinc content by more than 80% and causes almost complete loss of enzymatic activity. Activity can be restored by the addition of Zn2+, Mn2+, Fe2+, Co2+, or Cd2+.