The methylamine reactive site and protease inhibition in alpha 2-macroglobulin.

The inactivation of alpha 2M by the nucleophiles, NH4+, hydrazine, and methylamine, follows pseudo-first-order (second order, overall) rates. The rate of incorporation of nucleophiles however, is biphasic with the faster rate consistent with inactivation. Protease cleavage of alpha 2M is prevented by complete inactivation of alpha 2M by methylamine. Loss of protease cleavage is slower than methylamine incorporation but parallels inactivation. Our results are consistent with a complex model of sequential conformation changes leading to binding and inactivation of proteases by alpha 2M. Our results suggest that cross-linking of the protease to alpha 2M by aminolysis of the thiolester is not required for inactivation of the protease.