Table I. Estimated site occupancy of IgG1 and IgG4 Fc N-glycosylation sites from serum of healthy donors.

IgG1	Unmodified peptide area	Labeled D18O peptide area	% Occupied	IgG4	Unmodified peptide area	Labeled D18O peptide area	% Occupied
Donor 1A	3.59E+05	2.21E+08	99.84%	Donor 1A	1.31E+06	9.07E+08	99.86%
Donor 1B	2.84E+05	2.13E+08	99.87%	Donor 1B	1.31E+06	8.76E+08	99.85%
Donor 1C	2.90E+05	2.10E+08	99.86%	Donor 1C	1.00E+06	8.29E+08	99.88%
Donor 2A	2.55E+05	1.52E+08	99.83%	Donor 2A	4.92E+06	2.35E+09	99.79%
Donor 2B	2.79E+05	1.43E+08	99.80%	Donor 2B	3.66E+06	2.47E+09	99.85%
Donor 2C	2.24E+05	1.42E+08	99.84%	Donor 2C	4.99E+06	2.38E+09	99.79%
Donor 3A	1.03E+05	5.70E+07	99.82%	Donor 3A	1.33E+06	1.06E+09	99.87%
Donor 3B	7.66E+04	5.45E+07	99.86%	Donor 3B	1.31E+06	9.84E+08	99.87%
Donor 3C	7.20E+04	5.50E+07	99.87%	Donor 3C	2.60E+06	1.06E+09	99.75%
Donor 4A	8.96E+04	8.10E+07	99.89%	Donor 4A	7.37E+05	8.02E+08	99.91%
Donor 4B	7.95E+04	7.87E+07	99.90%	Donor 4B	5.88E+05	7.26E+08	99.92%
Donor 4C	7.41E+04	6.94E+07	99.89%	Donor 4C	2.48E+05	5.94E+08	99.96%

For each analysis, extracted ion chromatograms (EICs) of the unmodified peptide and the formerly glycosylated peptide labeled with ¹⁸O were generated, and the area under the curve was calculated. To estimate site occupancy, labeled (¹⁸O) peptide area was divided by the summed area of the labeled and unmodified peptide. Results of triplicate analyses are shown.