title: Homocysteine is a protein amino acid in humans. Implications for homocysteine-linked disease.
abstract: Homocysteine is thought to be a non-protein amino acid. However, in vitro studies suggest that homocysteine is likely to be incorporated by indirect mechanisms into proteins in living organisms. Here I show that homocysteine is a protein amino acid in humans. Homocysteine bound by amide or peptide linkages (Hcy-N-protein) is present in human hemoglobin, serum albumin, and gamma-globulins. 1 molecule of homocysteine per 1000 or 1670 molecules of methionine was present in hemoglobin or albumin, respectively. Other proteins, such as low density lipoprotein, high density lipoprotein, transferrin, antitrypsin, and fibrinogen, contained lower amounts of Hcy-N-protein. In human plasma, levels of Hcy-N-protein represented from 0.3 to 23% of total homocysteine. Thus, Hcy-N-protein is a significant component of homocysteine metabolism in humans, possibly contributing to adverse effects of homocysteine on human cells.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Medicine and Dentistry of New Jersey (UMDNJ)-New Jersey Medical School, International Center for Public Health, Newark, NJ 07101, USA. jakubows@umdnj.edu
authors.forename: Hieronim
authors.initials: H
authors.lastname: Jakubowski
mesh_terms: D001798:Blood Proteins / Q000032:analysis*; D006710:Homocysteine / Q000032:analysis / Q000378:metabolism*; D006801:Humans; D012709:Serum Albumin / Q000032:analysis
chemical_list: D001798:Blood Proteins; D012709:Serum Albumin; D006710:Homocysteine