Alpha 2-macroglobulin is the primary inhibitor of miniplasmin in vitro and in vivo in the mouse. Comparison with alpha 2-antiplasmin in simultaneous reaction experiments.

Miniplasmin reacted rapidly with purified human alpha 2-macroglobulin (alpha 2M). More than 98% of the complexes were stabilized by at least one covalent bond. The second-order rate constant for the reaction of alpha 2M with miniplasmin at 4 degrees C was 5.1 x 10(5) M-1.s-1. This value was determined by measuring the formation of covalent alpha 2M-125I-miniplasmin complex; however, the rate constant most likely reflects the bait-region cleavage step in the reaction mechanism. Miniplasmin bound primarily to alpha 2M when incubated at 37 degrees C with various mixtures of alpha 2-antiplasmin (alpha 2AP) and alpha 2M. A 2.4-fold molar excess of alpha 2AP was required to yield an equal distribution of proteinase between the two inhibitors. alpha 2M was the primary miniplasmin inhibitor in human and murine plasma (4 degrees C and 37 degrees C). The extent of covalent-bond formation with murine alpha 2M was approx. 96%. Intravenously injected miniplasmin cleared rapidly from the circulation of mice and was recovered principally in the liver. The catabolic pathway was distinctly different from that of miniplasminogen, which was sequestered mainly in the kidneys. The rate of miniplasmin clearance was much faster than that of purified alpha 2AP-miniplasmin complex, suggesting reaction with alpha 2M in vivo. This was confirmed in clearance competition experiments with alpha 2M-methylamine.