We studied some enzymes involved in the metabolism of sulfur-and selenium-containing amino acids. In this thesis, I introduce chiefly methionine γ-lyase and novel enzymes, selenocysteine β-lyase and NAD-L-penicillamine ADP transferase. We studied some properties, reaction mechanisms and suicide substrate reactions of their enzymes and functions of coenzymes involved. First, methionine γ-lyase, a pyridoxal-p dependent enzyme which was purified from Pseudomonas putida catalyzes α, γ-elimination and γ-replacement reactions of L-methionine and its derivatives, and also α, β-elimination and β-replacement reactions of S-substitude cysteines. The unique catalytic mechanism of methionine γ-lyase were studid using L-vinylglycine and is mechanism of inactivations were using suicide substrate, L-propargylglycine S-(N-methylthiocarbamoyl)-L-cysteine and L- 2-amino-4-chloro-4-pantanoate. The enzyme also catalyzes the rapid exchange of the α- and β-hydrogens of methionine and other amino acids with deuterium from solvents. From these studies, mechanism for α, γ-elimination and γ-replacement reactions by this enzyme have been proposed; α- and β-hydrogens of the substrate amino acids are initially removed, and then the γ-substitute is eliminated to yield a vinylglycine-pyridoxal-P intermediate, wthich is a common key intermediate in α, γ-elimination and γ-replacement reactions. In addition, the gene encoding this enzyme was cloned and the primary structure of the enzyme was deduced from its nucleotide sequences. The methionine γ-lyase gene was expressed in Escherichina coli. We found a part of an open reading frame (termed mde B) in the 3'-flanking region of the L-methionine γ-lyase gene, suggesting the presence of an operon involved in methionine catabolism. The deduced amino acid sequence of Mde B showed a high homology with the N-terminal region of the E1 component of pyruvate dehydrogenase complex from E. coli. We purified and characterized the novel α-ketobutyrate dehydrogenase E1 (Mde B) from E. coli transformant. Some of its properties were described. Second, We have found the presence of a new enzyme in various mammalian tissues, that cleaves specifically L-selenocysteine into L-alanine and H_2Se and named it selenocysteine β-lyase. The distribution of the enzyme in microorganisms and some properties of the bacterisl enzyme are described. The enzyme occurs widely in aerobic bacteria such as A. viscolactis. However, no significant activity is detected in yeasts and fungi. This enzyme requires pyridoxal-P as a colactor and is conpetitively inhibited by L-cysteine. Third, a starin of Bacillus sphaericus isolated from a local soil sample has been found to use DL -penicillamine (β, β-dimethyl-DL-cysteine) as the sole nitrogen source. Crude cell extract of the bacterium showed potent penicillamine -consuming activity only in the presence of NAD, which however was not used as an electron acceptor. Characterization of reaction products revealed that penicillamine was derivatized to a phosphoramide adduct with the ADP moiety of NAD, while nicotinamide-ribose group was relased and hydrolysed spontaneously to ribose and nicotinamide. The phosphoramide products, ADP-penicillamine, caused potent product inhibition on the purified enzyme, and adenylate deaminase was found to be effective in converting the inhibitory products into inosine-diphosphate-penicillamine and thereby mainteined the cataltsis for several hours. The novel enzyme, termed as NAD: penicillamine ADP transferase, was purified homogeneously. Its properties were described.