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 γ-globulins. 1 molecule of homocysteine per 1000 or 1670 molecules of
methionine was present in hemoglobin or albumin, respectively. Other proteins, such as low …