Homocysteine Thiolactone Detoxifying Enzymes and Alzheimer’s Disease

Elevated levels of homocysteine (Hcy) and related metabolites are associated with Alzheimer’s
disease (AD). Severe hyperhomocysteinemia causes neurological deficits and worsens behavioral and
biochemical traits associated with AD. Although Hcy is precluded from entering the Genetic Code by
proofreading mechanismsofaminoacyl-tRNAsynthetases, andthusisanon-proteinaminoacid,itcan
be attached to proteins via an N-homocysteinylation reaction mediated by Hcy-thiolactone. Because
N-homocysteinylation is detrimental to a protein’s function and biological integrity, Hcy-thiolactone
detoxifying enzymes—PON1, BLMH, BPHL—have evolved. This narrative review provides an
account of the biological function of these enzymes and of the consequences of their impairments,
leading to the phenotype characteristic of AD. Overall, accumulating evidence discussed in this
review supports a hypothesis that Hcy-thiolactone contributes to neurodegeneration associated with
a dysregulated Hcy metabolism.