Though in enzymatic reactions nicotinamide-adenine dinucleotide (NADH) serves as a reducing agent for aldehydes, via direct hydrogen transfer, rather extensive search has led to no aldehyde substrate reducible by 1.4-dihydropyridine in simple model systems operating in aqueous solution at ambient temperature.1 For horse liver alcohol dehydrogenase the Zn11 species present at each of the two NADH-containing active sites has been suggested to facilitate aldehyde reduction by polarization of the carbonyl group through direct interaction with the carbonyl oxygen.2 Though Creighton and Sigman3 have quite recently described a Zn11dependent reduction of 1,10-phenanthroline-2-carboxaldehyde by A-propyl-1,4-dihydronicotinamide (NPrNH), this system, being reported only in acetonitrile, is apparently restricted to aprotic solvents. Apparently searches for aldehyde substrates have never been directed to pyridoxal phosphate and analogs. This is rather surprising considering the fact that, as cofactors, NADH and pyridoxal phosphate cohabit in the same cell milieu. We report herein that pyridoxal phosphate (PLP), pyridoxal (PL), and 3-hydroxypyridine-4-aldehyde (PCHO) serve as suitable aldehyde substrates for reduction by the 1,4-dihydropyridines, NPrNH, and 2,6-dimethyl-3,5-dicarbethoxy-l,4-dihydropyridine (Hantzsch ester). Also, we find the reductions to be facilitated by metal ions in aqueous solution. Kinetic studies were carried out spectrophotometrically under No in Thunberg cuvettes employing two media: (a) neat boiling methanol (NPrNH at 354 nm, Hantzsch ester at 372 nm); and (b) 52.1 wt % (4) D. S. Auld and T. C. Bruice, ibid., 89, 2083Bruice, ibid., 89, (1967.(5) For a review of the work of E. E. Snell, D. E. Metzler, and others, see ref 1, Chapter 8.
