5-Methylnicotinamide-adenine dinucleotide. Kinetic investigation with major and minor isoenzymes of liver alcohol dehydrogenase and structural determination of its binary complex with alcohol dehydrogenase.

5-Methylnicotinamide-adenine dinucleotide and 3-cyano-5-methylpyridine-adenine dinucleotide was prepared from 5-methylthionicotinamide-adenine dinucleotide by chemical conversion. The 5-methylthionicotinamide-adenine dinucleotide was obtained by enzymic transglucosidation. Model compounds ascertained the structure. None of the dinucleotides methylated at C-5 was active with the major isoenzyme EE of horse liver alcohol dehydrogenase, but activity with 5-methylnicotinamide-adenine dinucleotide was measured with the minor isoenzymes. The binding of 5-methylnicotinamide-adenine dinucleotide to liver alcohol dehydrogenase, investigated by X-ray diffraction methods to 0.37-nm resolution, occurs with the pyridinium ring away from the active site as previously described for 3-iodopyridine-adenine and pyridine-adenine dinucleotides. A general conclusion on the use of inhibitors as tools for exploration of the active site is drawn.