Isocitrate dehydrogenase of the thermoacidophilic archaebacterium Sulpholobus acidocaldarius

The thermoacidophilic archaebacterium, Sulpholobus acidocaldarius has been found to possess both NAD‐ and NADP‐dependent isocitrate dehydrogenase activities. Evidence is presented to suggest that both enzymic activities are functions of the same protein: NAD+ and NADP+ compete with each other for the enzyme and do so with K i values equal to their K m values; thermal inactivation results in the loss of both activities at the same rate and copurification was observed on gel filtration, ion‐exchange chromatography and polyacrylamide gel electrophoresis. The evolutionary significance of this unique isocitrate dehydrogenase is discussed.