Identification and origin of Nε-homocysteinyl-lysine isopeptide in humans and mice

Homocysteine (Hcy) metabolites, Hcy-thiolactone and N-Hcy-proteins, have been linked to the pathology of human cardiovascular and neurodegenerative diseases. Hcy-thiolactone is generated in an error-editing reaction in protein biosynthesis when Hcy is selected in place of methionine by methionyl-tRNA synthetase. N-Hcy-protein, in which Hcy is linked via isopeptide bond to epsilon-amino group of a protein lysine residue, forms in a post-translational reaction of Hcy-thiolactone with proteins. Here, we identify a novel metabolite, N epsilon-Hcy-Lys, in human and mouse plasma, and show that this metabolite is elevated in genetic (cystathionine beta-synthase deficiency in humans and mice, methylenetetrahydrofolate reductase deficiency in mice) or dietary (high Met diet in mice) deficiencies in Hcy metabolism. We also show that N epsilon-Hcy-Lys is generated by proteolytic degradation of N-Hcy-protein in mouse liver extracts. Our data indicate that free N epsilon-Hcy-Lys is an important pathology-related component of Hcy metabolism in humans and mice.