The Reaction of Quaternary Pyridine Derivatives with Imidazoles*

Diphosphopyridine nucleotide reacts chemically with a number of reagents, including alkali (l), cyanide (2, 3), bisulfite ions (2, 3), dihydroxyacetone (4), dithionite ions (5), hydroxylamine (6), mercaptans (7), and aromatic amines (8). All of these reagents are nucleophilic agents which react on a positive center. It is generally assumed that these reactions involve the addition of the nucleophilic agent to the position of the nicotinamide moiety where the hydrogen addition occurs during enzymatic or chemical reduction, i.e. the para position (9). This assumption has proved valid for the cyanide (10) and dithionite (5) addition reactions. In one instance such an addition reaction seems to exist naturally. The binding of DPN to triosephosphate dehydrogenase from yeast or rabbit skeletal muscle results in an increased absorption in the region around 360 rnp (11). This results in a spectrum of the enzyme-DPN complex which is similar to that of the addition complexes enumerated above. The analogue of DPN, acetylpyridine-DPN, shows this effect in a much more outstanding manner (12). It has been assumed (11, 7) that this spectral band represents a mercaptan addition to the DPN. However, two other groupings on the protein could conceivably interact with DPN and thus be responsible for such a spectrum. These are the imidazole group of histidine and the phenolic group of tyrosine. The present paper describes the interaction which can exist between DPN and imidazoles.