Abstract
The dissociation constants of binary complexes formed between Mg2+ and pyridine nucleotide coenzymes have been measured by a number of independent methods. Spectrophotometric titration of NAD+ with Mg2+ in the presence of 8-hydroxyquinoline, and molecular exclusion chromatography of the coenzyme with Mg2+, agree on a value of about 20 mM for the dissociation constant of the NAD+. Mg2+ complex. Ligand competition between Mg2+ and DEAE-Sephadex indicates a similar value of the dissociation constant, for the corresponding complexes with NADH and NADP+. However fluorimetric titration of NADH with Mg2+ gives a much lower dissociation constant (0.01 mM), possibly because these experiments were necessarily performed in aqueous methanol. It is concluded that the Mg2+ probably binds to the pyrophosphate grouping in pyridine nucleotide coenzymes. The results are in accord with inferences drawn from enzyme kinetic studies.
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