Myotoxic
phospholipases A2 of class II are commonly found in the
venoms of crotalid snakes. As an approach to understanding their structure-activity relationship, diverse natural variants have been characterized biochemically and pharmacologically. This study describes a new myotoxic
phospholipase A2 homologue, isolated from the
venom of Atropoides (Bothrops) nummifer from Costa Rica. A. nummifer
myotoxin 1 is a basic
protein, with an apparent subunit molecular mass of 16 kDa, which migrates as a dimer in
sodium dodecylsulfate-
polyacrylamide gel electrophoresis under nonreducing conditions. It is strongly recognized by
antibodies generated against
Bothrops asper myotoxin II, by
enzyme-immunoassay. The toxin induces rapid myonecrosis upon
intramuscular injection in mice (evidenced by an early increase in plasma
creatine kinase activity), and significant
edema in the footpad assay. It also displays cytolytic activity upon cultured murine endothelial cells. The toxin (up to 50 microg) has no detectable
phospholipase A2 activity on egg yolk
phospholipids, and does not show an
anticoagulant effect on sheep platelet-poor plasma in vitro. N-terminal sequence determination (53
amino acid residues) demonstrated that A. nummifer
myotoxin II is a new Lys49 variant of the family of myotoxic, class II
phospholipases A2. Sequence comparison with other
phospholipases A2 revealed Asn53 as a novel substitution. In addition, this
myotoxin is the first Lys49 variant presenting Asn in its N-terminus. Consequently, these findings suggest that neither Ser1 or Lys53, usually found in this family of
proteins, are
essential amino acid residues for their myotoxic, cytolytic, or
edema-inducing effects.