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1. Inhibition by pyridine of reduction of NAD by ethanol in the presence of yeast alcohol dehydrogenase was studied at 25 degrees in 60mm-glycine buffer (K(+), pH9.3). 2. The apparent Michaelis constant for ethanol increases linearly and that for NAD increases non-linearly with pyridine concentration. 3. Rates, v, observed in the presence of pyridine are lower than the values calculated from the effect of pyridine on the two apparent Michaelis constants and are described by the expression V/v={1+5.8[pyridine]}x{1+0.016(1+124 [pyridine)]/[EtOH]}x{1+0.00019(1+3.3[pyridine]+110 [pyridine](2))/[NAD]}. 4. Mixed inhibitor studies with pyridine and N(1)-methylnicotinamide chloride in 40mm-pyrophosphate buffer (Na(+), pH8.2) indicated little interaction of pyridine with the ;pyridinium site' of the dehydrogenase (interaction constant, alpha, 2.1). 5. The possible competition of ethanol and pyridine for a zinc atom in the active centre of yeast alcohol dehydrogenase is discussed.

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The l-dimethylaminonaphthalene-5-sulphonyl group has been used extensively for fluorescent labelling of amino acids, peptides, and proteins; it has also been used to modify the pharmacological properties of a number of amines. The intense fluorescence of dimethylaminonaphthalene sulphonyl derivatives permits their detection and analysis on a millimicromolar scale (Neadle and PoUitt, 1965;Laurence, 1957).During a study of the metabolism of dimethylaminonaphthalene sulphonyl growth hormone, control rats were injected with l-dimethylaminonaphthalene-5-sulphonic acid and in 5-15 min. the urine was found to contain traces of l-aniinonaphthalene-5-sulphonic acid, together with much larger quantities of unchanged dimethyl acid and of an unidentified fluorescent acid that had less hydrophobic properties than the dimethyl acid, migrating with a lower R* on chromatography in organic solvents.The unknown metabolite has now been identified as the previously-undescribed 1-methylaminonaphthalene-5-sulphonic acid and this compound has been prepared by an unambiguous synthesis.Demethylation of the dimethyl sulphonic acid has been demonstrated in vitro with a rat-liver microsome fraction in the presence of a glucose-6-phosphate -glucose-6-phosphate dehydrogenase system to maintain a supply of reduced nico'.inamide-adenine dinucleotide phosphate. Purification of l-dimethylaminonaphthalene-5-sulphonic acid and synthesis of l-methylaminonaphthalene-5-sulphonic acid.Commercial l-dimethylaminonaphthalene-5-sulphonic acid (Fluka Co., "purum") contained several fluorescent impurities, including the methylamino derivative, which had R0 -56 in s-butanol-0-lN-NH^OH; 17:8, v/v; mobility 0-33 cm.^V.-ihr.-i on electrophoresis in 0-05M-tris citrate, pH 4-8. The impurities were removed by remethylation with methyl sulphate (Laurence, 1957); the recrystallised dimethyl sulphonic acid ran as a single fluorescent component (R^ 0-76) in s-butanol -ammonia and had a mobility of 0-26 cm.2V.~ihr. -' on electrophoresis in tris citrate.