ビタミンB_<12>の細胞内輸送と補酵素型合成系の比較生化学 : Euglena gracilisの知見を中心として

抄録

To elucidate the mechanisms of intracellular vitamin B_<12> (Cbl) transport and the coenzyme form synthesis in view of comparative biochemistry, the Euglena gracilis systems were studied. Euglena contained numerous Cbl-binding proteins in addition of Cbl-dependent enzymes. They were distributed in the extracellular, pellicular, cytosolic, mitochondrial, chloroplastic or microsomal fraction. Some of these binding proteins were purified to homogeneity and characterized. Euglena cytosolic binding proteins functioned in the intracellular Cbl transport. These observations suggest that Euglena can efficiently accumulate Cbl from a trace amount of Cbl in the environment using these Cbl-binding proteins. The Euglena system is different from the mammalian or the bacterial one. Activities of enzymes involved in the synthesis of the Cbl coenzymes were found in the Euglena mitochondria. Euglena aquacobalamin reductase was purified to homogeneity and characterized. The Euglena enzyme was a flavoprotein with Mr of 66,000 and was specific to NADPH and ON-Cbl. A novel enzyme, cyanocobalamin reductase (NADPH; CN-eliminating) required NADPH and FAD (FMN) for decyanation of CN-Cbl. Overall-conversion activity of OH-Cbl to AdoCbl was also found in the Euglena mitochondria, indicating that AdoCbl is synthesized only in the mitochondria. On the other hand, rat liver contained both NADH-and NADPH-linked aquacobalamin reductases. The NADH-linked enzyme was distributed in the mitochondrial outer and microsomal membranes. The distribution was identical to that of the NADPH-linked enzyme. The results suggest that the mammalian system for the coenzyme synthesis is distinct from the Euglena one.