Summary
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~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.