Conversion and Distribution of Cobalamin in Euglena gracilis z, with Special Reference to Its Location and Probable Function within Chloroplasts

Cobalamin is essentially required for growth by Euglena gracilis and shown to be converted to coenzyme forms promptly after feeding cyanocobalamin. Concentrations of coenzymes, methylcobalamin, and 5′-deoxyadenosylcobalamin, reached about 1 femtomole/10⁶ cells 2 hours after feeding cyanocobalamin to cobalamin-limited cells. Cobalamins all were bound to proteins in Euglena cells and located in subcellular fractions of chloroplasts, mitochondria, microsomes, and cytosol. Incorporated cobalamin into chloroplasts was localized in thylakoids. Methylcobalamin existed in chloroplasts, mitochondria, and cytosol, while 5′-deoxyadenosylcobalamin was in mitochondria and the cytosol, 2 h after feeding cyanocobalamin to Euglena cells. Quantitative alterations of methylcobalamin and 5′-deoxyadenosylcobalamin in chloroplasts suggest their important functions as coenzymes in this organelle. The occurrence of functional cobalamins in chloroplasts has not been reported in other photosynthetic eukaryotes.