PMID: 9192785Jun 15, 1997Paper

Cyanocobalamin [c-lactam] inhibits vitamin B12 and causes cytotoxicity in HL60 cells: methionine protects cells completely.

Blood
J H Matthews

Abstract

The [c-lactam] derivative of cobalamin antagonizes vitamin B12 in vivo. Therefore, we investigated its effects in tissue culture to develop a model in which to study vitamin B12-deficient hemopoiesis. HL60 cells were cultured in medium containing either methionine or L-homocysteine thiolactone, and various concentrations of 5-methyltetrahydrofolate or pteroylglutamic acid. In medium with L-homocysteine thiolactone, 5-methyltetrahydrofolate, and dialyzed serum, cyanocobalamin [c-lactam] caused cell death, reversible by additional vitamin B12. Pteroylglutamic acid did not prevent this cytotoxic effect. Methionine completely protected cells against cyanocobalamin [c-lactam] for periods of up to 4 months of culture, irrespective of the folate source. Cyanocobalamin [c-lactam] reversibly impaired the incorporation of 5-[14CH3]-tetrahydrofolate and [1-(14)C] propionic acid by intact cells, consistent with inhibition of methionine synthase and methylmalonyl-CoA mutase. A substantial proportion of 5-[14CH3]-tetrahydrofolate uptake could not be suppressed by methionine and may, therefore, have occurred outside of the methionine synthase pathway. These findings are the first indication that cyanocobalamin [c-lactam] antagonizes vitamin B...Continue Reading

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