title: Uptake of hydroxocobalamin by rat liver mitochondria. Binding to a mitochondrial protein.
abstract: Lysosome-free preparations of rat liver mitochondria take up hydroxo[57Co]cobalamin by a process which is dependent on mitochondrial swelling, rather than on energy or ion fluxes. The uptake system is saturable and unidirectional, leading to inside/outside concentration ratios of 17. The process also exhibits specificity: cyano[57Co]cobalamin is taken up less rapidly and to a lesser extent than hydroxocobalamin; methylcobalamin and adenoslcobalamin inhibit hydroxocobalamin uptake markedly, while cyanocobalamin does not. The [57Co]cobalamin ([57Co]Cbl) taken up is bound to a mitochondrial protein whose apparent molecular weight is 120,000 by Sephadex G-150 chromatography. Double reciprocal plots of bound [57Co]Cbl versus medium [57Co]Cbl concentration yield estimates for bound Cblmax of 29 pmol/mg of protein and for Kd is 8.2 muM. We conclude that mitochondrial uptake of cobalamins occurs via the diffusion of free cobalamins into the mitochondria and their subsequent binding to a high affinity mitochondrial protein(s) which we propose to be the source of the unidirectional character, the saturability, and the specificity of the uptake system.
mesh_terms: D000135:Acid Phosphatase / Q000378:metabolism; D000263:Adenylate Kinase / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D004072:Digitonin / Q000494:pharmacology; D005696:Galactosidases / Q000378:metabolism; D005969:Glutamate Dehydrogenase / Q000378:metabolism; D006879:Hydroxocobalamin / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000201:enzymology; D008247:Lysosomes / Q000187:drug effects / Q000201:enzymology; D008297:Male; D008930:Mitochondria, Liver / Q000187:drug effects / Q000378:metabolism*; D008933:Mitochondrial Swelling / Q000187:drug effects; D011485:Protein Binding; D011506:Proteins / Q000378:metabolism*; D051381:Rats; D014805:Vitamin B 12 / Q000494:pharmacology
chemical_list: D011506:Proteins; D005969:Glutamate Dehydrogenase; D000263:Adenylate Kinase; D000135:Acid Phosphatase; D005696:Galactosidases; D004072:Digitonin; D014805:Vitamin B 12; D006879:Hydroxocobalamin