Cyanocobalamin [c-lactam] inhibits vitamin B12 and causes cytotoxicity in
HL60 cells: methionine protects cells completely
JH Matthews
Department of Medicine, Queen's University, Kingston, Ontario, Canada.
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 B12 in vitro and causes cell death from
methionine deficiency. The model should be valuable for investigating the
biochemical pathology of vitamin B12-deficient hemopoiesis. The results
suggest that methylfolate is not trapped when methionine synthase is
inhibited in HL60 cells, but they do not disprove the methylfolate trap
hypothesis as applied to normal blood cells.
Volume 89,
Issue 12,
pp. 4600-4607,
06/15/1997
Copyright © 1997 by The American Society of Hematology
