Alpha 2M Complexes Research Articles

Inhibition of proteases by alpha 2-macroglobulin. The role of lysyl amino groups of trypsin in covalent complex formation.

The lysyl amino groups of bovine trypsin were covalently modified by acetylation, succinylation, or reductive methylation. The enzymatically active derivatives were still capable of reaction with alpha 2-macroglobulin (alpha 2M), although to a lesser extent than native enzyme. The resulting enzyme-alpha 2M complexes, however, were much more susceptible to dissociation by sodium dodecyl sulfate than complexes formed with unmodified trypsin. The bound modified enzymes could be released from the alpha 2M complex with an excess of native thrombin. In addition, anhydrotrypsin displaced methyl trypsin from its complex and the anhydro derivative was bound in its place. The data provide evidence for two types of noncovalent intermediates; those formed from lysyl-modified enzymes show proteolysis of the alpha 2M to the nominal 85,000 fragment, whereas anhydrotrypsin forms a complex with apparently intact alpha 2M chains. A model is proposed for the reaction of alpha 2M with proteases in which one or both of these noncovalent intermediates is formed. Conversion of this form(s) to a stable covalent complex requires unmodified lysyl amino groups on the enzyme, suggesting that these groups may form a covalent bond with the inhibitor, possibly at the site at which methylamine binds.

Generation of an esterolytic and kinin-forming kallikrein-alpha2-macroglobulin complex in human serum by treatment with acetone.

Kinin-forming and esterolytic activity in human citrate plasma has been activated by treatment of the plasma with acetone. By far most of the esterolytic activity if not all of it was recovered in an alpha2-macroglobulin (alpha2M) kallikrein complex (SI) which was characterized and purified by chromatography. Little if any esterolytic activity was present which could be ascribed to free plasma kallikrein. The alpha2M-kallikrein complex had kinin-releasing activity though much less than free plasma kallikrein, relative to esterolytic potency. This explains that a considerable fraction of the kinin-forming potential of acetone-activated plasma resides in free plasma kallikrein although it represents only a very small portion of the total kallikrein store. Like free plasma kallikrein the alpha2M complex releases kinin from LMW-kininogen less efficiently than from HMW, in systems of purified components. In whole plasma, the efficiencies change: whereas plasma kallikrein is rapidly inactivated by endogenous inhibitors, the alpha2M complex is protected from further inactivation and capable of releasing kinin continuously if slowly, attacking also LMW-kininogen after HMW-kininogen has been consumed by free kallikrein. While the alpha2M-complex in this respect differs functionally from free plasma kallikrein and explains earlier observations suggesting the presence of two kininogenases, it seems doubtful now that two truly different kininogenases exist in human plasma. The results suggest that acetone predominantly inactivates full inhibitors of kallikrein such as C1INH whereas alpha2M is somewhat more resistant and (pre-)-kallikrein even more. Depending on the time and temperature of acetone treatment one obtains more or less total kallikrein and varying proportions of free to bound enzymes. It is likely that acetone does not turly trigger an activation of prekallikrein but supports spontaneous activation by slowing down the control of the feedback reinforcement of this activation, by damaging inhibitors.