Structure of the Human Adult Hemoglobin Minor Fraction AIb by Electrospray and Secondary Ion Mass Spectrometry

Hemoglobin AI,, is a minor hemoglobin component from human hemolysate (less than 0.5% of total hemoglobin) whose structure has never been established. It was purified and studied by mass spectrometry. Electrospray ionization of its abnormal &chain indicated a 70-Da mass increase. Separation of the tryptic digest by reversed-phase liquid chromatography revealed an abnormal @T1 peptide. Cesium ion bombardment ionization produced a protonated molecular ion at mlz 1022.516, showing an additional C3H202 residue to normal BT1. The amino acid sequences of both abnormal and normal BT1 peptides were found identical by comparison of their collision activation spectra. Time course hydrolysis of abnormal BT1 indicated a rapid loss of the modifying group, leading to normal BT1. At least, mild treatment with acidic methanol showed an additional methylated site, comparatively with normal BT1. All these results are consistent with a ketiminelinked pyruvic acid at the amino end of the &chain of hemoglobin.