Characterization of a novel streptokinase produced by Streptococcus equisimilis of non-human origin

Streptokinases are proteins with plasminogen activator activity produced by certain hemolytic streptococci. We previously identified equine streptococcal isolates which produced streptokinases (ESKs) that bound both human and equine plasminogen but only readily activated equine plasminogen (14). This property was exploited to purify a representative ESK produced by Streptococcus equisimilis strain 87-542-W. Affinity chromatography with human plasminogen resulted in the isolation of a M(r) approximately 49,000 molecule with two isoforms. This ESK was subsequently compared to well characterized streptokinases (HSKs) that efficiently activate human plasminogen. Differences in streptokinases were identified in the highly conserved amino-terminal amino acid sequence, peptide maps, and antigenic properties, and these differences were supported by DNA hybridization studies. These results indicate that the family of proteins identified as streptokinases has much greater diversity than previously appreciated.