• Blood cells;
  • peripheral;
  • plasminogen activation;
  • fibrinolysis;
  • proteases

The α isoform of enolase is a candidate plasminogen receptor on U937 monocytoid cells [Miles, L. A., Dahlberg, C. L., Plescia, J., Felez, J., Kato, K. & Plow, E. F. (1991) Biochemistry 30, 1682–1691]. In the present study, an enolase-related molecule was detected on the surfaces of peripheral blood monocytes and neutrophils by fluorescence-activated cell sorting. A mRNA transcript encoding a unique membrane form of an enolase-related molecule was not detected by Northem-blotting and primer-extension analyses, consistent with the cell-surface protein being authentic α-enolase. Both the α and β isoforms of purified enolase, bound plasminogen with an affinity similar to that of the cell surface. Moreover, immunopurified α-enolase enhanced plasminogen activation by tissue plasminogen activator and blocked the binding of plasminogen to α2-antiplasmin, mimicking functions arising from the association of plasminogen with cells. The interaction of the enolase isoforms with plasminogen was dependent upon recognition of the C-terminal lysyl residue of the enolases by the lysine-binding sites of plasminogen, as the interaction was blocked by (a) peptides with C-terminal lysine residues and (b) an antibody to the C-terminal aspect of enolase. A monoclonal antibody was developed, characterized and utilized to quantify the enolase molecules present on the surface of U937 cells. A substantial number of molecules, 1.8×106/cell, was present, accounting for approximately 10% of the plasminogen-binding capacity of these cells. These studies clearly establish the role of enolase as a cell-surface plasminogen-binding site with profibrinolytic functions.