AMINO-ACID-SEQUENCES OF A HETERODIMERIC NEUROTOXIN FROM THE VENOM OF THE FALSE HORNED VIPER (PSEUDOCERASTES FIELDI)

The main toxic component of the venom of the false horned viper, Pseud ocerastes fieldi, is a heterodimeric neurotoxin composed of a basic su bunit, Cb II, and one of two acidic subunits, either Cb I alpha or Cb I beta. The nontoxic acidic subunit increases the toxicity of the basi c subunit. Both subunits have phospholipase A(2) (PLA(2)) amino acid s equences. Cb I alpha and Cb I beta themselves are inactive towards pho sphatidylcholine and when complexed with Cb II promote a delay in the onset of phospholipase activity of Cb II. Cb I alpha and Cb I beta do hydrolyze the synthetic substrate, 3-octanoyloxy-4-nitrobenzoic acid, but at <1% the rate of Cb II. Comparisons of the amino acid sequences of Cb II and Cb I alpha with the corresponding acidic and basic subuni ts of other heterodimeric neurotoxins show high amino acid sequence id entity. Some of the amino acids which are different between the acidic and basic subunits are in highly conserved sequences in their respect ive types of PLA(2). This suggests that these amino acid changes in th e conserved regions are important for the structure and function of th e heterodimeric proteins.