Purification and Properties of Inosinic Acid llehydrogenase from E’scherichia coZi*

Inosinic acid dehydrogenase was prepared from a derepressed mutant of Escherichia coli. Although the reaction catalyzed formally resembles those catalyzed by chicken liver xanthine dehydrogenase and related enzymes, this enzyme is devoid of any flavin, iron complex, or molybdenum component, and the kinetics indicate necessity for concomitant binding of substrate and DPN. The enzyme is absolutely dependent on Kf or NH4+, which are competitive with the inhibitor Mg++. A series of purine nucleotides inhibits competitively with substrate with no evidence of cooperative interactions. The enzyme is quite unstable and can be separated into several distinct but enzymatically equivalent fractions by centrifugation or electrophoresis.