Alpha 2-macroglobulin (alpha 2M) and related proteins share the function of binding host or foreign peptides and particles, thereby serving as humoral defense barriers against pathogens in the plasma and tissues of vertebrates. In human alpha 2M, several reactive sites including high-affinity sites for zinc, transglutaminase cross-linking sites, and reactive sites derived from the activated thiol ester can mediate reversible or irreversible capture of proteins of diverse biological functions. Alpha 2M interacts and captures virtually any proteinase whether self or foreign, suggesting a function as a unique "panproteinase inhibitor." Activation of alpha 2M generates novel binding sites, which mediate complex formation with cytokines and other peptides. Direct evidence of physical association of cytokines with activated alpha 2M indicated its role as biological response modifier in cell cultures. A mechanism commonly referred to as "clearance of activated alpha 2M" involves Ca(2+)-dependent binding to a specific cell surface receptor, a member of the low-density lipoprotein receptor supergene family, that mediates cellular uptake by endocytosis and delivery to endosomes and lysosomes. The peptide binding function of alpha 2M, therefore, may also be viewed as a mechanism that allows targeting of biologically active peptides to different cell types expressing the alpha 2M receptor. Internalized complexes may be dispatched into different pathways of endocytic/lysosomal pathways in a cell type-specific manner. In addition, bioactive peptides bound to alpha 2M may dissociate in the process of intracellular ligand sorting, thereby modulating cell function, or remain bound and share the catabolic fate of alpha 2M. The diversified and probably programmed binding functions of alpha 2M indicate that in addition to its role in trapping proteinases, it has other biological activities that remain to be fully defined. That alpha 2M may function as a binding and carrier protein with targeting characteristics is predicted from 1) the known functions of alpha 2M, and 2) the similarity of the fate of alpha 2M with proteins whose significance in targeting and intracellular trafficking has been studied in more detail.

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Growing evidence suggests that moderately elevated levels of homocysteine are associated not only with arterial thrombosis and atherosclerosis but also with venous thrombosis as well. We have reviewed recent studies that indicate that homocysteine inhibits several different anticoagulant mechanisms that are mediated by the vascular endothelium. The protein C enzyme system appears to be one of the most important anticoagulant pathways in the blood. Homocysteine inhibits the expression and activity of endothelial cell surface thrombomodulin, the thrombin cofactor responsible for protein C activation. Homocysteine inhibits the antithrombin III binding activity of endothelial heparan sulfate proteoglycan, thereby suppressing the anticoagulant effect of antithrombin III. Homocysteine also inhibits the ecto-ADPase activity of human umbilical vein endothelial cells (HUVECS). Because ADP is a potent platelet aggregatory agent, this action of homocysteine is prothrombotic. Homocysteine also interferes with the fibrinolytic properties of the endothelial surface because it inhibits the binding of tissue plasminogen activator. Homocysteine stimulates HUVEC tissue factor activity. We have found that lipoprotein(a) [Lp(a)] also stimulates HUVEC tissue factor activity. The combination of Lp(a) plus homocysteine induced more tissue factor activity than either agent alone. These disruptions in several different vessel wall-related anticoagulant functions provide plausable mechanisms for the occurrence of thrombosis in hyperhomocysteinemia.