BETA-141-LEU IS NOT DELETED IN THE UNSTABLE HEMOGLOBIN ATLANTA-COVENTRY BUT IS REPLACED BY A NOVEL AMINO-ACID OF MASS 129 DALTONS

Reinvestigation of the structure of the beta-chain of Hb Atlanta-Coventry (beta-75 Leu --> Pro, beta-141 Leu deleted) confirmed the presence of two abnormalities; however, analysis of the aberrant beta(Co)14 tryptic peptide by liquid secondary ion mass spectrometry indicated that the beta-141 Leu (mass 113 daltons) was not deleted but replaced by a novel amino acid of mass 12 9 daltons. The new amino acid in peptide beta(Co)14 was uncharged at pH 6.5, more hydrophilic than leucine and susceptible to cleavage by both chymotrypsin and carboxypeptidase A. We propose that the new residue is likely to be hydroxyleucine and that it results from post-translational oxidation of beta-141 Leu as a consequence of perturbation of the haem environment caused by the beta-75 Leu --> Pro mutation in the E helix (E19). This proposal is entirely consistent with recent DNA analysis which showed that beta(At-Co) was not the product of a third beta-globin gene and that neither of the two beta-globin genes, beta(A) nor beta(Atlanta), contained a deletion of the beta-141 Leu codon. We have subsequently found this modified amino acid at position beta-141 in two other unstable haemoglobins, both of which involve mutations on the haem side of the E helix.