The M gamma chain of human fetal hemoglobin is an A gamma chain with an in vitro modification of gamma 141 leucine to hydroxyleucine.
We have reanalyzed the structure of the gamma T-15 peptide from the minor M gamma chain of human hemoglobin (Hb) F. Amino acid analysis confirmed that the Leu 141 residue was missing from position 9 of this peptide, and liquid secondary ion mass spectrometry indicated that it was replaced, not by methionine (residue mass 131) as previously believed, but by an amino acid of mass 129. By analogy with the recently reported oxidation of the corresponding leucine at position gamma 141 of the unstable Hb Atlanta, it appears that the M gamma chain also results from the oxidation of gamma 141 to hydroxyleucine (residue mass 129). The finding that the proportion of the M gamma chain increased when red cell lysates were prepared with carbon tetrachloride prompted us to reinvestigate the oxidation mechanism involved in the formation of beta 141 hydroxyleucine in Hb Atlanta. Oxidation of the beta 141 residue could be detected when carbon tetrachloride was used in the lysis protocol, while conversion of oxyhemoglobin to carbon monoxyhemoglobin prior to carbon tetrachloride treatment prevented oxidation. It therefore appears that the hydroxylation of Leu 141 is not an in vivo process in the circulating red cell. Perhaps leucine at position 141 of the beta, gamma, and delta chains (and at position 136 of the alpha chain), which forms a contact with heme and is located directly across the heme plate from the E helix, is oxidized to hydroxyleucine at a very low rate forming minute amounts of modified chains; this process is accelerated by treatment with agents such as carbon tetrachloride and prolonged exposure to air.