Use of combined mass spectrometry methods for the characterization of a new variant of human hemoglobin: The double mutant hemoglobin villeparisis β77(EF1) His → Tyr, β 80 (EF4) Asn → Ser.
Hemoglobin Villeparisis was found during a systematic measurement of glycated hemoglobin. Electrospray mass spectra of the globin indicate an apparently unchanged molecular weight within the error range (0.01%). The tryptic digest of the β chain shows a chromatographically abnormal βT-9 peptide. The mass-to-charge ratio value of its [M+H](+) ion, as measured by liquid secondary ionization mass spectrometry, is one mass unit lower than that of the normal βT-9. However, the electrospray mass spectrum of this peptide exhibits mainly a doubly charged ion, whereas the normal βT-9 gives a triply charged ion. None of the allowed single amino acid substitutions for a 1-u shift down (Glu → Gln, Asp → Asn, or Asn → Ile) can explain the suppression of one protonation site. This can be due only to the replacement of the internal histidine by a nonbasic residue. Thus at least two amino acid exchanges occur within the same peptide: one involves the internal histidine, and the sum of the mass shifts is -1 u. Consideration of the βT-9 sequence and taking account for the genetic code rules, the only possibility was (11)His → Tyr (+26 mass shift) associated with (14)Asn → Ser (-27 mass shift). This conclusion was consistent with the tandem mass spectrum of the [M+H](+) ion and was further confirmed by chemical microsequencing.