title: Liver betaine-homocysteine S-methyltransferase activity undergoes a redox switch at the active site zinc.
abstract: 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.
authors.affiliation: Area de Fisiología, Facultad de Medicina, Universidad de Cádiz, Plaza Falla 9, 11003 Cádiz, Spain. carmen.castro@uca.es
authors.forename: Carmen
authors.initials: C
authors.lastname: Castro
authors.forename: Norman S
authors.initials: NS
authors.lastname: Millian
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Garrow
mesh_terms: D050940:Betaine-Homocysteine S-Methyltransferase / Q000737:chemistry*; D001665:Binding Sites; D002384:Catalysis; D004789:Enzyme Activation; D006801:Humans; D008099:Liver / Q000201:enzymology*; D010084:Oxidation-Reduction; D011485:Protein Binding; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D015032:Zinc / Q000737:chemistry*
chemical_list: D013438:Sulfhydryl Compounds; D050940:Betaine-Homocysteine S-Methyltransferase; D015032:Zinc