Cobalamin metabolism in cultured human chorionic villus cells

Abstract

Cobalamin (Cbl, vitamin B12) metabolism was analyzed in cultures of human chorionic villus (CV) cells obtained at 9-10 weeks of gestation. CV cells were shown to synthesize transcobalamin II (TCII) and to possess a high affinity receptor for that molecule. The cells bound and internalized radioactive cyanocobalamin (CN[57Co]Cbl) complexed to TCII. This internalized CN[57Co]Cbl was found to be converted to both methylCbl and adenosylCbl, the two intracellular coenzyme forms of Cbl, and bound to the two known intracellular Cbl requiring enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase. Both enzyme systems were found to be functional in the intact cell by demonstrating the incorporation of the radioactive label from both [14C]CH3-tetrahydrofolate and [14C]propionate into acid insoluble products. MS activity was also detected in lysed cell material. CV cells were shown not to be auxotrophic for methionine since they were able to utilize homocysteine in place of methionine for cell division. Since CV cells are capable of performing many of the complex events associated with Cbl metabolism, it may be possible to use these cells to diagnose genetic defects of Cbl metabolism.

Different uptake of cobalamin (vitamin B12) by astrocytes and oligodendrocytes isolated from rat spinal cord

“…This difference leads us to hypothesize a direct influence of serum component(s) on the Cbl uptake of these cells. Futhermore, the two-peak uptake by oligodendrocytes can be ascribed to the metabolic transformation of the [Cyano-14 C]cyano-Cbl molecule to the coenzymatic forms of the vitamin (methyl(Me)-Cbl and Adenosyl(Ado)-Cbl) (Begley et al, 1993): in this case, when [Cyano-14 C]cyanoCbl enters the cell, the [Cyano-14 C]group can be removed by a cleavage system and the molecule converted to Ado-Cbl and Me-Cbl (Begley et al, 1993;Quadros and Jacobsen, 1995). The concomitant existence of an efflux system that releases the excess [Cyano-14 C]cyano-Cbl not used by the cell in this metabolic transformation justifies the nil uptake at 12 h (Quadros and Jacobsen, 1995).…”

Experimental Observations on the Biological Significance of Hydrogen Sulfide in Carotid Body Chemoreception

“…Consistent with this effect of exogenous sulfi de the hydroxyl form of vitamin B12 or hyroxocobalamin, which reacts with sulfi de (Truong et al 2007 ; as well as it reacts with HCN; Borron et al 2007 ) reverses the effect produced by exogenous sulfi de. Further, hydroxocobalamin which enters inside cells and mitochondria (Begley et al 1993 ;Buccellato et al 2004 ;see Depeint et al 2006 ) and would purportedly react with endogenously generated sulfi de, increases basal and hypoxic-stimulated accumulation of cAMP, implying that endogenous sulfi de physiologically inhibits cAMP. It should be emphasized, however, that the inhibition exerted by endogenous sulfi de, is quantitatively comparable in normoxia and hypoxia, implying that it does not interfere with the intimate mechanism used by hypoxia to activate the cycling enzyme.…”