Purification and Characterization of Malate Dehydrogenase from Cryptococcus neoformans

The NAD-dependent malate dehydrogenase (EC 1.1.1.37) was purified from Cryptococcus neoformans, a basidiomycetious yeast that is an opportunistic pathogen of AIDS patients. The purified enzyme was a dimer of 35 kDa subunits that exhibited uncompetitive substrate inhibition by oxalacetate, typical for mitochondrial malate dehydrogenases from other sources. Product inhibition studies indicated an ordered sequential kinetic mechanism, with pyridine dinucleotide being the substrate that binds to the free enzyme form. Unique aspects of this malate dehydrogenase were inhibition by zinc ion, competitive versus malate with Ki of 30 μM, and inhibition by heparin. Heparin inhibition was competitive versus either NAD or malate, with Ki of 0.35 μM. Heparin molecules of nominal molecular weight of 30,000 or 3000 were equally effective inhibitors. A model is presented to explain the high affinity of the enzyme for heparin.