The multiplicity of cardiotoxins from Naja naja atra (Taiwan cobra) venom

Four novel cardiotoxins were isolated from Naja naja atra (Taiwan cobra) ve nom by successive separation on a SP-Sephadex C-25 column and a reverse pha se column. Amino acid sequences of the cardiotoxins were determined by Edma n degradation and carboxypeptidase digestion. It shows that these cardiotox ins comprise 60 amino acid residues. Comparative analyses on the amino acid sequences of cardiotoxins from the venoms of N. naja atra and other Naja s pecies indicated that amino acid substitutions of cardiotoxin isoforms freq uently occurred at positions 7-11, 27-32 and 45-47. The hypervariable segme nts encoded by the second and third exon of cardiotoxin genes are located a t or near the tips of loop structure of cardiotoxin molecules. These result s, together with the suggestions that the residues at the tips of cardiotox ins' loop structure were involved in the manifestation of the biological ac tivities of cardiotoxins, reflect that the preferential mutations may contr ibute to alterations in the function of cardiotoxin molecules. Analysis on the secondary structure of pre-mRNAs of N. naja atra cardiotoxin 4 gene and N. naja sputatrix cardiotoxin 3 gene has shown that the hypervariable regi ons of the tron 2 pertain to form intra-exon pairings and are not involved in the formation of intron-exon pairings. Since the pairings of splice site s and gene architecture were supposed to be associated with intron-exon rec ognition, it is likely that the preferred loci of mutations occurring with the evolution of cardiotoxin genes would not affect the processing of cardi otoxin precursors. (C) 2000 Elsevier Science Ltd. All rights reserved.