The molecular weight and oligomerization of rabbit thrombomodulin as assessed by sedimentation equilibrium

Lubrol-solubilized rabbit thrombomodulin has been examined by equilibrium sedimentation in buffers that include sufficient D2O to make the detergent neutrally buoyant. Data were acquired at rotor speeds from 12,000 to 28,000 rpm from two thrombomodulin preparations, at protein concentrations from 0.01 to 0.07%, and in buffer containing 0.01 to 0.23% Lubrol. Examination of the data from different rotor speeds shows that the thrombomodulin exists as a heterogeneous mixture containing monomer (Mr 65,000), trimer, and higher oligomers. The oligomers do not equilibrate over the time scale of the experiment. The weight fraction as monomer varies from preparation to preparation, and appears to be independent of detergent concentration. Thus, experimenters should be cautious when interpreting binding or kinetic results obtained under similar buffer conditions.