抄録
Vitamin B 12 (B 12 ) is synthesized only in certain bacteria． B 12 is concentrated mainly in the body of the higher predatory organisms in the natural food chain． Animal-derived foods such as meats， shellfish， egg， and milk are good dietary B 12 sources． In addition， a B 12 compound was contained abundantly in edible cyanobacteria． The B 12 compound present in edible cyanobacteria has been identified as pseudovitamin B 12 ， an inactive corrinoid compound for humans． Therefore， this study was attempted to clarify the physiological function of pseudovitamin B 12 in cyanobacteria． The full-length of Spirulina B 12 -dependent methionine synthase was cloned into E．coli to clarify the physiological function of pseudovitamin B 12 in cyanobacteria． The recombinant Spirulina enzyme showed a higher affinity for methylpseudovitamin B 12 than methylcobalamin as a cofactor， indicating that cyanobacateria can utilize pseudovitamin B 12 as the cofactor for B 12 -dependent methionine synthase．
It was further examined whether pseudovitamin B 12 exists in dietary sources other than edible cyanobacteria． As a result， a large amount of pseudovitamin B 12 was detected in turban shell and abalone， herbivorous sea snails． The content of B 12 in seaweeds which herbivorous sea snails consume as the food was very low， suggesting that pseudovitamin B 12 detected in herbivorous sea snails might be derived from cyanobacteria adhering to seaweeds．
