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 K2+ and Na2+ were neither essential for activity nor stability of the protein. The enzyme was extraordinarily thermostable.