The capacity of alpha2-macroglobulin to inhibit an exogenous protease is significantly increased in critically ill and septic patients.

ABSTRACT

The image of alpha2-macroglobulin is based on a paradigm evolved in the 1970s. During this decade alpha2-macroglobulin was shown to irreversibly entrap and thereby inhibit a maximum of two proteases. Additional binding of nonproteolytic proteins, i.e., inflammatory mediators and growth factors, is dependent on the conformational status of alpha2-macroglobulin. It was our aim to clarify whether the interaction of nonproteolytic proteins with alpha2-macroglobulin during inflammatory conditions might also modulate the capacity of alpha2-macroglobulin to inhibit proteases. To explore this possibility, bromelain, an exogenous protease, was titrated against plasma of critically ill or septic patients, whose pathophysiological conditions are defined by a massive release of inflammatory mediators. The stoichiometry of bromelain inhibition by alpha2-macroglobulin was quantified by caseolytic activity assays. The maximal alpha2-macroglobulin/bromelain inhibition ratios were significantly increased (16 and 18 in the two patient groups, P < 0.01) as compared with control groups (12 with purified alpha2-macroglobulin and 14 in healthy volunteers). The increase of alpha2-macroglobulin inhibition capacity in patients was paralleled by the appearance of a large signal on Western blots, suggesting the formation of additional complexes. Our results demonstrate alpha2-macroglobulin to have more flexibility than had been thought, and it may thereby contribute to a shift in a 30-year-old paradigm.