The
venoms of the Naja species are known to be cytotoxic. This toxicity has been attributed to the presence of small nonenzymatic
polypeptides of 60
amino acid residues, designated as
cardiotoxins or
cytotoxins. We investigated the cytotoxic potency of Naja nigricollis
venom fractions and isolated another type of cytotoxic component which is even more potent than
cardiotoxins. This cytotoxic compound, which was designated as
nigexine, was purified to homogeneity and its amino acid sequence was determined.
Nigexine is a basic
phospholipase A2 consisting of a single chain of 118
amino acids. A detailed investigation of the cytotoxic effects on epithelial FL cells, C-13T
neuroblastoma cells, and promyelocytic
leukemia HL 60 cells revealed that
nigexine not only altered cell viability but also prevented cell proliferation. This is a property that was specific to
nigexine since other
phospholipases A2 from various sources had no detectable cytotoxic activity. The cytotoxic activity of
nigexine was not dependent on the presence of
divalent cations, unlike its enzymatic activity. In particular, the cytotoxic activity of
nigexine was identical in the presence or absence of either 2 mM Ca2+ or Sr2+, or 6 mM
EDTA. We also present evidence based on chemical modifications that cytotoxic activity was not correlated with enzymatic activity. Thus, modification with parabromophenacyl
bromide totally abolished the enzymatic activity of
nigexine, which nevertheless retained 6-20% of the cytotoxicity of native
nigexine. Conversely, treatment with
cyanogen bromide gave a compound that retained 7% of the enzymatic activity of the parent molecule but was devoid of detectable cytotoxicity.