Expediting Rare Variant Hemoglobin Characterization by Combined HPLC/Electrospray Mass Spectrometry

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Abstract
Microscale analysis of a variant hemoglobin (Hb) has been achieved by combination of high performance liquid chromatography (HPLC) and electrospray mass spectrometry (ESMS) and the method should be almost universally applicable. We have eliminated preparative scale HPLC of globin chains and semi-preparative HPLC of proteolytic digests which had been used prior to mass spectrometry. Use of microbore HPLC columns reduced the time required for analysis substantially and solvent usage by 100x. Molecular masses of intact globins and masses and sequence information of tryptic peptides could be obtained without collecting and separately analyzing chromatographic fractions. As an example of the use of these methods, we report the characterization of an unknown hemoglobinopathy case that was finally authenticated as Hb P-Galveston [beta 117(G19)His-->Arg], using the following sequence of analyses: 1) ESMS of complete hemolysate, 2) analytical HPLC of globin chains, 3) combined microbore HPLC/ESMS of globin chains to determine their molecular masses, 4) cysteine derivatization and tryptic digestion of mixture of all globins, followed by microbore separation of the peptides, molecular mass determination, and generation of fragmentation patterns allowing confirmation of amino acid sequences. This four-part strategy should allow characterization of almost all variant Hbs. Exceptions would be mutations in regions of globin chains which give rise to small (< four residues) tryptic peptides, either normal or produced by addition of new tryptic sites and mutations that introduce only minute difference in molecular weight (MW) of tryptic peptides. Since only 10% of each separated peptides is mass analyzed, 90% is available for collection and further structural identification (e.g. by tandem MS or Edman sequencing) if the identity is still in doubt.

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