Figure 1.
Schematic of the One-Carbon cycle.
Vitamin B12 is a co-factor in the transfer of methyl-groups (CH3−) from folate to methionine for use in situ methylation of deoxycytidine (dC) to in 5-methyldeoxycytidine (5 mdC) in DNA. Choline, betaine and methionine can be derived from the diet or synthesized in vivo. SAM: S-adenosylmethionine; SAH: S-adenosylhomocysteine.
Table 1.
Basic Biochemical Characteristics of Subjects.
Table 2.
Effect of vitamin B12-deficiency on % 5 mdC1 for DNA extracted from coagulated blood clots2 and uncoagulated EDTA-blood cell pellets3.
Table 3.
Effect of vitamin B12-deficiency on % 5 mdC1 and [3 H]methyl-acceptance capacity2 for DNA extracted from coagulated blood clots3 and uncoagulated EDTA-blood cell pellets4.
Figure 2.
Plot of % 5-methyldeoxycytidine versus [3H]methyl-acceptance capacity of DNA extracted from coagulated blood clots.
DNA samples were from (•) vitamin B12-deficient (plasma vitamin B12<148 pmol/L) or (□) vitamin B12-replete subjects (plasma vitamin B12>148 pmol/L). % 5-methyldeoxycytidine: 5-methyldeoxycytidine as a percentage of total deoxycytidine in DNA digests as measured by LC-MS/MS (see Methods). [3 H]Methyl-acceptance: [3 H] (cpm) from [3 H-methyl]SAM incorporated into 250 ng of DNA in the presence of Sssi DNA methyltransferase.
Figure 3.
Putative mechanisms for the conversion of 5 mdC to dC in DNA.
Reactions: 1) 5 mdC nucleases [91]; 2) 5 mdC glycosylase [92], [93], [94]; 3) 5 mdC deaminase [84], [87], [95], [96]; 4) thymine-DNA glycosylases [97]; 5) 5 mdC hydroxylase [63], [64]; 6) 5 hmC glycosylase [98]; 7) AP endonuclease/phosphodiesterase [99], [100]; 8) DNA polymerase/DNA ligase [100]; 9) 5 hmdC dehydroxymethylase [88], [89]; 10) DNA demethylase [101], [102], [103].