Shear stress facilitates oxidation of methionine residues buried in the VWF A domains. Plasma VWF was incubated with 50μM H₂O₂ and 25nM MPO under either shear or static conditions. The percentage of methionine converted to the sulfoxide Met(O) was analyzed by LC-MS/MS. The results represent the means ± SD of at least 3 experiments. ASA of the methionine side chain was analyzed on 3D structures of each domain. In the ribbon structures of the A domains, methionine side chains are depicted in ball-and-stick form. In the absence of shear stress, little oxidation was detected at the completely buried methionine residues (A1: M1303, M1304, and M1385; A2: M1521 and M1606; A3: M1761). Their oxidation was greatly increased under shear, indicating that the 3 VWF A domains change their conformations under shear, exposing the buried methionines and facilitating their oxidation.