Diazotized 3-aminopyridine adenine dinucleotide has been found to modify four sulfhydryl groups per molecule of enzyme during the complete inactivation of yeast alcohol dehydrogenase. The reaction of sulfhydryl groups was indicated by titration studies with 5,5'- dithiobis (2-nitrobenzoic acid) as well as isolation and quantification of the cysteinyl derivative released by acid hydrolysis of the modified enzyme. The cysteinyl derivative was identified as S-(3-pyridyl) cysteine. Authentic S-(3-pyridyl) cysteine was synthesized and structurally characterized for these studies. Diazonium-sulfhydryl reactions were demonstrated for a number of diazonium derivatives with cysteine, homocysteine, glutathione and mercaptoethanol at 0-4° and neutral pH. Second order rate constants were determined in reactions of these sulfhydryl compounds with diazotized l-methyl-3-aminopyridinium chloride, diazotized 3-aminopyridine adenine dinucleotide and diazotized 3-aminopyridine adenine dinucleotide phosphate. Chemical studies of the diazonium-sulfhydryl reaction of diazotized p-aminobenzoic acid with cysteine indicated the initial formation of a diazomercaptide which can then decompose to yield the thioether. / Ph. D.