used in this study were prepared from NeoRecormon (EPO
beta) drug substance material from Roche Diagnostics GmbH (Penzberg,
Germany). Endoproteinase GluC, guanidinium hydrochloride (Gua-HCl),
tris(hydroxymethyl)aminomethane (Tris), 1,4-dithiothreitol (DTT),
and NAP-5 gel filtration columns were from Sigma-Aldrich/Merck KGaA
(Darmstadt, Germany). Ammonium hydrogen carbonate, acetonitrile (ACN),
formic acid (FA), and iodoacetic acid (IAA) were obtained from Fisher
Scientific International Inc. (Pittsburgh, PA, USA).