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The metabolism of diacetyl (2,3-butanedione), acetoin (3-hydroxy-2-butanone), and 2,3-butanediol, which are metabolites of acetaldehyde was quantitatively investigated using rat liver homogenate, liver perfusion, and in vivo experiments. Diacetyl and acetoin were reduced to 2,3-butanediol in these experiments, but acetoin and 2,3-butanediol were scarcely oxidized to diacetyl, indicating that the reduction reaction to 2,3-butanediol from diacetyl occurs actively in rat liver. The formation of acetoin from diacetyl required either NADH or NADPH as a reductant, while the reduction of acetoin to 2,3-butanediol required NADH. Acetoin and 2,3-butanediol were more readily accumulated than diacetyl in brain tissue.

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This is a report investigating the methylglyoxal (MG) bypass in animals, by which D-lactate is produced from triosephosphate via MG. Rats were made diabetic using streptozotocin or starved for 72 h. D-Lactate and various metabolites related to it, such as L-lactate, pyruvate, methylglyoxal, glucose, and inorganic phosphate, were measured in the blood plasma, liver, and skeletal muscle of the rats. Diabetic and starved rats had significantly higher levels of D-lactate in plasma, liver, and skeletal muscle compared with the control group. In contrast, pyruvate levels in plasma, liver, and skeletal muscle was markedly lower than normal in diabetic and starved rats. L-Lactate level lowered markedly in plasma, liver, and skeletal muscle of starved rats and elevated in liver of diabetic rats. Differences between plasma L-lactate level for diabetes and control were not significant. MG level was significantly elevated in plasma and depressed in livers and muscles of starved rats as well as livers of diabetic rats. Hepatic glycerol content was markedly increased in those states. Enzyme activities related to D- and L-lactate, such as pyruvate kinase, phosphofructokinase, aldolase, and glyoxalase I, were measured in the livers of these rats. Pyruvate kinase activity decreased in these states, but other enzyme activities showed no significant changes. D-Lactate was much more excreted than L-lactate in the urine of diabetic and fasted rats compared with normal rats.

A DNA fragment containing the Saccharomyces cerevisiae CYS3 (CYII) gene was cloned. The clone had a single open reading frame of 1,182 bp (394 amino acid residues). By comparison of the deduced amino acid sequence with the N-terminal amino acid sequence of cystathionine 'y-lyase, CYS3 (CYI1) was concluded to be the structural gene for this enzyme. In addition, the deduced sequence showed homology with the following enzymes: rat cystathionine y-lyase (41%), Escherichia coli cystathionine -y-synthase (36%), and cystathionine 13-lyase (25%). The N-terminal half of it was homologous (39%o) with the N-terminal half of S. cerevisiae O-acetylserine and O-acetylhomoserine sulfhydrylase. The cloned CYS3 (CYIJ) gene marginally complemented the E. coli metB mutation (cystathionine 'y-synthase deficiency) and conferred cystathionine 'y-synthase activity as well as cystathionine y-lyase activity to E. coli; cystathionine 'y-synthase activity was detected when O-succinylhomoserine but not O-acetylhomoserine was used as substrate. We therefore conclude that S. cerevisiae cystathionine 'y-lyase and E. coli cystathionine -y-synthase are homologous in both structure and in vitro function and propose that their different in vivo functions are due to the unavailability of O-succinylhomoserine in S. cerevisiae and the scarceness of cystathionine in E. coli.Saccharomyces cerevisiae biosynthetic pathways of the sulfur-containing amino acids cysteine and methionine are shown in Fig. 1. Cysteine is synthesized via two pathways. One consists of serine O-acetyltransferase (SATase) (EC 2.3.1.30) and O-acetylserine sulfhydrylase (OAS SHLase) (EC 4.2.99.8). This is the autotrophic pathway for sulfur utilization, because inorganic sulfur is converted to organic sulfur. It is similar to the enteric bacterial and plant cysteine biosynthetic pathways (for reviews, see references 9 and 37). In enteric bacteria and plants, cysteine sulfur is converted to homocysteine sulfur via transsulfuration consisting of cystathionine y-synthase (-y-CTSase) (EC 4.2.99.9) and cystathionine P-lyase (1-CTLase) (EC 4.4.1.8), and the resultant homocysteine is used for methionine biosynthesis. The same takes place in S. cerevisiae. However, it should be stressed that while S. cerevisiae uses O-acetylhomoserine (OAH) for cystathionine biosynthesis, enteric bacteria and plants use O-succinylhomoserine (OSH) and O-phosphohomoserine, respectively.The other S. cerevisiae cysteine biosynthetic pathway is reverse transsulfuration, which consists of cystathionine ,B-synthase (P-CTSase) (EC 4.2.1.22) and cystathionine y-lyase (-y-CTLase) (EC 4.4.1.1). In this pathway, homocysteine sulfur is converted to cysteine sulfur. This pathway is present in mammals but in neither enteric bacteria nor plants (for a review, see reference 10). Mammals synthesize homocysteine exclusively by demethylation of methionine and convert homocysteine sulfur to cysteine sulfur. In this respect, reverse transsulfuration is a part of the heterotrophic pathway for sulfur utilization. S. cerevisiae syn...