Studies on coenzyme III I. L-cysteinesulfinic dehydrogenase and its prosthetic group

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.