Ref ID : 3862
F. Sarhan, M. Houde, and J.P. Cheneval; The Role of Vitamin B12 Binding in the Uptake of the Vitamin by Euglena gracilis. J.Protozool. 27(2):235-238, 1980
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The uptake of [57Co]B12 (cyanocobalamin) by Euglena gracilis strain Z (ATCC12716) occurred in 2 distinct phases- an initial rapid phase followed by a slower secondary phase. This secondary phase appeared after the saturation of the binding sites involved in the initial rapid phase and was energy-dependent and completely inhibited by 2,4-dinitrophenol, KCN and sodium azide. The subcellular localization of labeled cyanocobalamin taken up by the cell was mostly contained in the chloroplast fraction. The time course and the saturation kinetics of B12 uptake by purified chloroplast fraction indicated that this fraction and the intact cell had a similar affinity for the vitamin B12. This suggested that the chloroplasts contained the binding sites for vitamin B12 and might regulate the uptake process in the intact cell. The kinetic properties of the overall B12 uptake mechanism suggested that the initial phase represent the binding of vitamin B12 to the available sites on the chloroplast. The secondary phase may represent the de nova synthesis of new binding sites.