The rate of gross conformational change of a,-macroglobulin (a,M) during its proteinase trapping was directly determined for the first time using time-resolved X-ray solution scattering. Decrease of radius of gyration was observed under pseudo-first-order conditions with excess proteinases, which exhibited a monophasic timecourse. The rate constants were 0.5 f 0.1 s-' and 0.8 f 0.2 s-' for the reaction with chymotrypsin and trypsin, respectively. There was no concentration dependence of the observed rate constants. Therefore, the rate-limiting step of the gross conformational change was not the bimolecular encounter reaction between a,M and proteinases, which requires a new proposal of pre-trapping of proteinases before the gross conformational change.

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