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1. 1. L-Cysteinesulfinic dehydrogenase, the enzyme which catalyzes the dehydrogenation of L-cysteinesulfinic acid, requires a coenzyme for activity. The coenzyme requirement is not fulfilled by known vitamins, coenzymes, or related substances, but boiled yeast extract is a good source of the cofactor. 2. 2. The new coenzyme, provisionally named coenzyme III, has been extensively purified from baker's yeast. When the dehydrogenase oxidizes cysteinesulfinic acid in the presence of the purified coenzyme, the latter is reduced to a substance with an absorption spectrum characteristic of dihydropyridine nucleotides. Upon addition of L-cysteic acid, the reduced spectrum disappears, indicating reversal of the dehydrogenation. 3. 3. The purified coenzyme is rapidly destroyed by 0.1 N acids at 100° and by nucleotide pyrophosphatase but is not acted upon by 5-nucleotidase or phosphodiesterase. The heavy metal salts of coenzyme III are water soluble. In charcoal chromatography it behaves very much like nicotinamide mononucleotide. 4. 4. On the basis of its behaviour to chemical reagents and to hydrolytic enzymes, the structure of coenzyme III is tentatively suggested to be nicotinamide-ribose-(5)-pyrophosphate. 5. 5. Some of the salient properties of L-cysteinesulfinic dehydrogenase have been investigated. 6. 6. On the basis of a detailed study of the electron transport from coenzyme III to O2, it is concluded that a diaphorase (or cytochrome reductase), concerned with the reoxidation of coenzyme III, is present in the extracts in the apoenzyme form. This protein requires a second new coenzyme, which is not identical with FMN or FAD, but is likely to be a flavin. The latter cofactor is present in yeast extract and is readily separated from coenzyme III. 7. 7. It is pointed out that coenzyme III also functions as the prosthetic group of dehydrogenases of animal origin and that it appears to be of wide distribution.

The fact that the d -α-hydroxy acid dehydrogenase is formed best under anaerobic conditions of growth and disappears on environmental adaptation of the cells to O2, while l (+) lactic dehydrogenase is absent in anaerobic cells and is synthesized during O2 adaptation, has repeatedly led to suggestions in the literature that d -α-hydroxy acid dehydrogenase is a precursor of l (+) lactic dehydrogenase. In the present study it is shown that the disappearance of d -α-hydroxy acid dehydrogenase during O2 adaptation is the consequence of the fact that the enzyme is rapidly destroyed in yeast cells once active growth ceases and that the destruction is as rapid on anaerobic incubation of the cells as aerobically. Thus the destruction of d -α-hydroxy acid dehydrogenase can proceed without any synthesis of l (+) lactic dehydrogenase. Further, under suitable conditions, extensive synthesis of l (+) lactic dehydrogenase occurs without the disappearance of d -α-hydroxy acid dehydrogenase . These facts and the extensive differences in constitution and catalytic properties between d -α-hydroxy acid dehydrogenase and l (+) lactic dehydrogenase suggest that a precursor-product relationship between the two enzymes is highly unlikely.

The cytoplasmic membrane of the H 37 Ra strain of Mycobacterium tuberculosis has been isolated free of cell wall. These membrane preparations contain very small quantities of cytochromes c , b and cytochrome oxidase. The cytochrome c is not extracted by any method attempted. The cytochrome b is reducible only by dithionite and is believed not to be involved in the direct transfer of electrons during the oxidation of NADH by these preparations. The NADH oxidase activity of the membrane is inhibited by high concentrations of cyanide and also by 2-( n -heptyl)-4-hydroxyquinoline- N -oxide (HQNO). The cytochrome oxidase of the membrane contains both cytochromes a and a 3 and is present in low concentrations relative to cytochrome c . The cytochrome a 3 component was identified by characteristic complexes with both CO and cyanide and shows a γ-band absorption maximum at a slightly lower wavelength than the cytochrome oxidase of mammalian mitochondria (442 nm vs. 445 nm). The functional activity of the cytochrome oxidase is indicated by the inhibition of reoxidation of reduced cytochromes c and a in the presence of cyanide.

1. 1. A soluble enzyme preparation, obtained from Proteus vulgaris by ultrasonic disintegration, in the presence of suitable cofactors, catalyzes the rapid oxidation of L-cysteinesulfinic acid. The metabolism of the latter compound is shown to proceed by two simultaneous and competing pathways. In pathway A it is oxidized to cysteic acid and the latter is transaminated with α-ketoglutarate. Pathway B is initiated by transamination of cysteinesulfinate with α-ketoglutarate (or oxaloacetate). The resulting β-sulfinylpyruvic acid is desulfinated to pyruvate and SO3= and the latter is oxidized to SO4=, while α-ketoglutarate is regenerated under the influenceof a TPN-specific glutamic dehydrogenase. 2. 2. Three new enzymes are described: L-cysteinesulfinic dehydrogenase (the enzyme responsible for the formation of cysteic acid), oxaloacetic-cysteinesulfinic and α-ketoglutaric-cysteinesulfinic transaminases. The properties of the latter are discussed in some detail. 3. 3. β-Sulfinylpyruvic acid, an analogue of oxaloacetic acid, which has not hitherto been implicated in intermediary metabolism, has been accumulated by transaminase action. This compound is desulfinated to SO3= and pyruvate, under the influence of Mn++, analogously to the β-decarboxylation of oxaloacetic acid by Mn++. The desulfination is a rapid, non-enzymic reaction, as is the oxidation of SO3= to SO4= by Mn++. 4. 4. The possibility is discussed that enzymes may exist for the desulfination reaction and that the reversal of their action (SO2 fixation into pyruvate) may be a mechanism for the incorporation of inorganic sulfur into organic linkages. 5. 5. L-Cysteinesulfinic dehydrogenase requires a previously unrecognized, pyridine nucleotide coenzyme. 6. 6. The applicability of the metabolic scheme to intact bacterial cells and to animal tissues is discussed.