Liver betaine-homocysteine S-methyltransferase activity undergoes a redox switch at the active site zinc

Arch Biochem Biophys. 2008 Apr 1;472(1):26-33. doi: 10.1016/ Epub 2008 Jan 31.


Using a redox-inert methyl acceptor, we show that betaine-homocysteine S-methyltransferase (BHMT) requires a thiol reducing agent for activity. Short-term exposure of BHMT to reducing agent-free buffer inactivates the enzyme without causing any loss of its catalytic zinc. Activity can be completely restored by the re-addition of a thiol reducing agent. The catalytic zinc of BHMT is bound by three thiolates and one hydroxyl group. Thiol modification experiments indicate that a disulfide bond is formed between two of the three zinc-binding ligands when BHMT is inactive in a reducing agent-free buffer, and that this disulfide can be readily reduced with the concomitant restoration of activity by re-establishing reducing conditions. Long-term exposure of BHMT to reducing agent-free buffer results in the slow, irreversible loss of its catalytic Zn and a corresponding loss of activity. Experiments using the glutamate-cysteine ligase modifier subunit knockout mice Gclm(-/-), which are severely impaired in glutathione synthesis, show that BHMT activity is reduced about 75% in Gclm(-/-) compared to Gclm(+/+) mice.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Betaine-Homocysteine S-Methyltransferase / chemistry*
  • Binding Sites
  • Catalysis
  • Enzyme Activation
  • Humans
  • Liver / enzymology*
  • Oxidation-Reduction
  • Protein Binding
  • Sulfhydryl Compounds / chemistry*
  • Zinc / chemistry*


  • Sulfhydryl Compounds
  • Betaine-Homocysteine S-Methyltransferase
  • Zinc