True values of Michaelis constants of the @entity1110 -specific @entity9275 dehydrogenase from @entity28887 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 @entity1110 at fixed concentrations of 1.0-0.2mm and @entity69399 at fixed concentrations of 2.0-0.125mm. The response of apparent V(max.) to salt concentration was highly dependent on fixed substrate concentration in solutions of @entity2827 but much less so in solutions of @entity4337 . 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- @entity4337 ), and also in 1.0m- @entity2827 , the reaction mechanism is ordered with @entity1110 the first substrate added and @entity1110 the last product released. In 0.25m- @entity2827 , however, the mechanism is different and is probably non-sequential. In 4.0m- @entity2827 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 @entity2827 is more complex than the corresponding inhibition by @entity4337 . Gel-filtration experiments indicated that at very low salt concentrations the enzyme has an apparent molecular weight of about 70800. In ;physiological' concentrations of @entity4337 the enzyme appears to be a dimer (mol.wt. 122000-135000) and, in 1.0-4.0m- @entity2827 , it behaves as a trimer or tetramer (mol.wt. 224000-251000). A preliminary method of purifying the enzyme is described.

Properties of halophil XXXX -specific @entity9275 dehydrogenase. True Michaelis constants, reaction mechanisms and molecular weights.

@entity1110 :: ('MESH:D009249', 'Chemical') :: ['NADP(+)', 'NADPH'] @entity28887 :: ('2242', 'Species') :: ['Halobacterium salinarium'] @entity69399 :: ('CHEBI:15563', 'Chemical') :: ['threo-d(s)-(+)-isocitrate'] @entity2827 :: ('MESH:D012965', 'Chemical') :: ['sodium chloride'] @entity4337 :: ('MESH:D011189', 'Chemical') :: ['potassium chloride'] @entity9275 :: ('MESH:C034219', 'Chemical') :: ['isocitrate']
@entity1110 :: (MESH:D009249,Chemical) :: ['nicotinamide-adenine dinucleotide phosphate']
