HIGH-ACCURACY MOLECULAR-WEIGHT DETERMINATION AND VARIATION CHARACTERIZATION OF PROTEINS UP TO 80-KU BY IONSPRAY MASS-SPECTROMETRY

A quadrupole mass spectrometer with an ionspray interface was used to measure the molecular weight (MW) of proteins up to 80,000 u. With the improvements in instrument calibration by a statistical averaging method and in data analysis by a gaussian curve-fitting method, precision of MW determination as high as 12 ppm was achieved with equine myoglobin (MW 16,950.4 +/- 0.2 u). Exact MW determination of three components in cerato-ulmin revealed that the two minor ones had lost amino acid residues Ser and Ser-Asp, respectively, from the major component (MW 7618.4 +/- 0.2 u). MW classification of eight components in the Fab fragment of a monoclonal antibody revealed that one set of four had MW approximately 47,540 u and the other approximately 47,640 u. The MW difference of 100.2 +/- 0.6 u between fragment 1 and 2, attributed to inhomogeneous cleavage at the Fab C-terminus, was probably due to one additional Thr in 1. The MW of bovine serum albumin (BSA) was found to be 66,431.5 +/- 1.3 u, approximately 164 u higher than the calculated sequence MW, most probably because of the incorrectness in the previously reported BSA amino acid sequence. The MW of human serum transferrin (79,556.8 +/- 1.7 u) was shown to be 4414 u higher than the sequence MW, pointing to a glycosylation of 22.7 sugar units in this protein. The greater complexity in bovine serum transferrin (MW 78,030.5 +/- 1.8 and 78,326 +/- 3.3 u for the two major components) was correlated with the heterogeneity in the glycosylation.