Extracellular metal-free corrinoids from Rhodopseudomonas spheroides

Rhodopseudomonas spheroides when grown on a medium deficient of cobalt excretes significant amounts of descobaltocorrinoids into the culture broth. If grown in the presence of 4 microM CoCl2 only intracellular cobalt-containing corrinoids are detected. The extracellular corrinoids have been identified as hydrogenobyrinic acid c-amide and hydrogenobyrinic acid a,c-diamide which are accompanied by varying amounts of the corresponding 13-epicorrinoids. The latter, which could not be separated in the metal-free form, are artifacts arising from the excreted descobaltocorrinoids under the alkaline conditions of the culture broth. The red metal-free corrinoids have been converted into the corresponding cobalt-containing corrinoids and were characterized by their CD, UV, 1H NMR, and 13C NMR spectra. Insertion of cobalt into the fraction of hydrogenobyrinic acid c-amide has yielded small amounts of a cobalt-containing corrinoid, for which the structure of 18,19-didehydrocobyrinic acid c-amide is suggested. A 14C-labeled specimen of hydrogenobyrinic acid a,c-diamide was obtained by feeding [14C]acetate to growing cultures of R. spheroides. Experiments with broken cell systems of Propionibacterium shermanii and R. spheroides have indicated that metal-free corrinoids are not precursors of the cobamides. The results are discussed with respect to the final steps of the biosynthesis of vitamin B12.