Phenotyping of α-1-Antitrypsin by liquid chromatography-high resolution mass spectrometry

More than seventy-five isotypes of α-1-antitrypsin (AAT) have been described. To assess risks associated with AAT deficiency, isotype identification is necessary. Isoelectric focusing (IEF) is traditionally used for isotype differentiation, however, IEF has limited scope since it is a manual procedure that is not suitable for automation, and antitrypsin variants must differ in net charge in order to be resolved. In comparison, mass spectrometric assays are easily automated and offer a more complete solution for characterization of proteins. To capitalize on these advantages, we have developed a qualitative top-down liquid chromatography-mass spectrometry (LC-MS) method for selective phenotyping of AAT. This technique requires no sample pretreatment, and has the potential for use in routine clinical diagnostics. We have validated our LC-MS results against both DNA sequencing and IEF. Thus far, this method has identified the AAT variants PLowell, S and Z, as well as unique fragments shared by different M alleles. Its high selectivity is indirectly illustrated by the detection of a variant carrying the amino acid substitution p.Ala308Ser, which cannot be visualized by IEF.