Malate Dehydrogenase Isolated from Extremely Halophilic Bacteria of the Dead Sea. 1. Purification and Molecular Characterization

The complete purification of malate dehydrogenase (EC 1.1.1.37) from extremely halophilic bacteria of the Dead Sea is described. The purification procedure includes (a) precipitation by ammonium sulfate, (b) fractionation on Sepharose 4B using a decreasing concentration gradient of ammonium sulfate, (c) gel permeation chromatography on Sephadex G-100, (d) chromatography on hydroxylapatite, and (e) affinity chromatography on 8-(6-aminohexyl)amino-NAD⁺-Sepharose at 4.26 M NaCl. The absorption and fluorescence spectra of the native and denatured enzyme were measured, and the extinction coefficient at 280 nm in 4.26 MNaCl was found to be 0.803 cm² mg^-1. The amino acid composition analysis showed an excess of 10.4 mol % of acidic amino acids. The apparent specific “volume” ϕ′ of the active enzyme at 4.26 M NaCl was found to be 0.680 ± 0.015 mL/g. The molecular weight of the native enzyme was found to be 84 000 ± 4000 determined in 4.26 M NaCl from equilibrium sedimentation data. The molecular weight of the subunits is 39 000 as measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Thus, the native enzyme is composed of two subunits.