[Ribonucleotide reductase in Propionibacterium shermani]

The cell-free extract of Propionibacterium shermanii was found to contain B12-dependent ribonucleotide reductase. The extract of the cells grown under the conditions of the inhibited synthesis of vitamin B12 reduces ribonucleotides with the participation of B12-independent enzyme. The synthesis of B12-dependent apoenzyme of ribonucleotide reductase is partially maintained under these conditions. Both enzymes reduce preferably ribonucleoside diphosphates. The reducing agent of nucleotides in vitro is lipoic acid or dithiothreitol, in the B12-dependent pathway, and NADPH and thioredoxin, in the B12-independent pathway. Only B12-independent ribonucleotide reductase requires Mg2+ ions. Vitamin B12 in the coenzyme form inhibits the activity of B12-independent enzyme.