Purification and Characterization of 3-isopropylmalate Dehydrogenase from a Thermoacidophilic Archaebacterium Sulfolobus Sp. Strain 7

3-Isopropylmalate dehydrogenase was purified (about 2000-fold) to homogeneity for the first time from an archaebacterium, Sulfolobus sp. strain 7. The enzyme showed an apparent molecular mass of about 110 kDa by gel filtration and a single 36-kDa polypeptide band on SDS-PAGE, suggesting tri- or tetrameric structure. The pI value was 6.9. The N-terminal amino acid sequence was similar to enzymes from other sources. The enzyme activity was greatly stimulated by the presence of Mn2+, Cd2+, Mg2+, or Co2+. In contrast to 3-isopropylmalate dehydrogenase from other sources, monovalent cations such as K+ and Na+ were neither essential for activity nor stability of the protein. The enzyme was extraordinarily thermostable.