Plasmhogen Activators from the Saliva of Desmodus rotundus (Common Vampire Bat): Unique Fibrin Specificity

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Abstract
The saliva of D. rotundus contains at least four plasminogen activators (PAs) which all require fibrin as a cofactor. D. rotundus salivary PAs (DSPAs) exhibit a sequential array of structural motifs such as "Finger" (F), "EGF" (E), "Kringle" (K) and "Protease" (P) which was elucidated by cDNA cloning and sequencing. The respective domain organizations are: FEKP (DSPA alpha 1 and DSPA alpha 2), EKP (DSPA beta) and KP (DSPA gamma). In all four forms the plasmin-sensitive site of tPA is obliterated, indicating that they function as single-chain enzymes. DSPA alpha 1 differs from alpha 2 by amino acid substitutions found mainly in the F, E and K domain, 11% of the total sequence. DSPA beta and gamma, while being closely related to alpha 2, still exhibit 2 and 13 amino acid exchanges, respectively. These sequence heterogeneities, together with results of Southern blot hybridization experiments, strongly suggest that the four DSPA mRNA species originate from different genes. All four forms of DSPA have been expressed in animal cell culture and DSPA alpha 1 was chosen for a detailed pharmacological characterization. In vitro DSPA alpha 1 activity is enhanced 50,000-fold in the presence of fibrin, whereas the activity of single chain tPA is only enhanced 100-fold. At equally effective thrombolytic doses DSPA causes lower bleeding incidence in a rat mesenteric vein model and exhibits high potency, clot selectivity, and speed in the dissolution of fibrin embolized into the lung of anesthetized rats. In the copper coil-induced dog coronary heart infarction model, at doses that achieve patency at equal rates, reocclusion is significantly less frequent than with tPA. These results indicate that DSPA alpha 1 may be a safer and more efficacious thrombolytic agent than the PAs currently in clinical use.