UPTAKE OF COBALAMIN BY EUGLENA-MITOCHONDRIA

Cobalamin uptake by Euglena mitochondria is a biphasic process, consis ting of energy-independent cobalamin-binding to mitochondrial membrane s and energy-dependent active transport. The energy-dependent phase of cobalamin uptake is not dependent on mitochondrial respiration, but o n the presence of ATP within the mitochondrial matrix. The dissociatio n constant of the energy-independent cobalamin-binding reaction is est imated to be 0.45 nM. Inhibition of the mitochondrial cobalamin uptake by a variety of cobalamin analogues indicates that Euglena mitochondr ia have an absolute requirement for the complete cobalamin molecule wi th an a-axial ligand (the cobalt-coordinated nucleotide) and an intact b-propionamide side-chain. Thus, the Euglena mitochondrial cobalamin uptake system is highly specific for the cobalamin structure. The coba lamin taken up by the Euglena mitochondria cannot be exchanged with ex ogenous cobalamin. All of the mitochondrial cobalamin is associated wi th three proteins with molecular masses of > 700,000 (16.3%), 160,000 (7.4%), and 35,000 (76.3%). They occur in the soluble fraction of mito chondria, suggesting that these cobalamin-binding proteins or cobalami n-dependent enzymes play an important role in cobalamin accumulation a nd metabolism within the mitochondria.