True values of Michaelis constants of the NADP+-specific isocitrate dehydrogenase from Halobacterium salinarium were not very different from those of the apparent constants reported by Aitken et al. (1970). The true constants were affected by salt in a similar manner to that of the apparent constants obtained with NADP+ at fixed concentrations of 1.0–0.2mm and threo-ds-(+)-isocitrate at fixed concentrations of 2.0–0.125mm. The response of apparent Vmax. to salt concentration was highly dependent on fixed substrate concentration in solutions of sodium chloride but much less so in solutions of potassium chloride. At several levels the results emphasize the difficulty of generalizing about the salt relations of a halophil enzyme without adequate attention to substrate concentration. The enzyme has at least two different reaction mechanisms depending on salt concentration. In its ‘physiological’ form (i.e. in 1.0m-potassium chloride), and also in 1.0m-sodium chloride, the reaction mechanism is ordered with NADP+ the first substrate added and NADPH the last product released. In 0.25m-sodium chloride, however, the mechanism is different and is probably non-sequential. In 4.0m-sodium chloride with low concentrations of either fixed substrate, there was evidence of a co-operative action of the variable substrate. The evidence suggests that salt participates in the reaction mechanism in two ways: one is the reversible addition to the enzyme in a manner analogous to that of a substrate; the other is dead-end complex-formation. The relative contributions of these two types of reaction determine whether salt activates or inhibits the enzyme. In addition, the inhibition caused by high concentrations of sodium chloride is more complex than the corresponding inhibition by potassium chloride. Gel-filtration experiments indicated that at very low salt concentrations the enzyme has an apparent molecular weight of about 70800. In ‘physiological’ concentrations of potassium chloride the enzyme appears to be a dimer (mol.wt. 122000–135000) and, in 1.0–4.0m-sodium chloride, it behaves as a trimer or tetramer (mol.wt. 224000–251000). A preliminary method of purifying the enzyme is described.

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