Vitamin B12 metabolism in a photosynthesizing green alga, Chlamydomonas reinhardtii.

To elucidate vitamin B12 (cobalamin) metabolism in green algae, Chlamydomonas reinhardtii was labeled with radioactive cobalamin for 4 days. The Chlamydomonas cells did not require cobalamin for growth, but nevertheless took up and accumulated exogenous cobalamin. The level of the accumulated cobalamin reached a maximum (about 20 fmol/10(6) cells) at day 1 of the cell growth. The cobalamin accumulated by the 4-day cells was partly converted to the coenzymes (5'-deoxyadenosylcobalamin, 20.7% and methylcobalamin, 4.8%). The cells contained NADH- and NADPH-linked aquacobalamin reductases (35.9 and 31.5 nmol/min per mg protein, respectively) involved in the synthesis of the coenzymes. Most of the accumulated cobalamin was associated with two heat-labile macromolecules; the coenzymes synthesized were recovered in the macromolecular fraction with a molecular weight of 230,000, while non-coenzyme forms (cyanocobalamin and hydroxocobalamin) predominated in that with a molecular weight of 40,000. Evidence for de novo biosynthesis of cobalamin was also obtained in the alga, indicating that it has both ability to synthesize cobalamin and take up the exogenous vitamin.