Sequence characterization of cardiotoxins from Taiwan cobra: isolation of a new isoform

Cardiotoxins (or called cytotoxins) are a group of very basic polypeptides present in some snake venoms, which are especially abundant in Formosan cobras. Several cardiotoxins with distinct pharmacological and biochemical properties were isolated and purified from the Taiwan cobra venom (Naja naja atra) by employing sequentially preparative-scale cation-exchange chromatography on TSK CM-650 coupled with the improved separation of toxin components on reversed-phase HPLC based on their hydrophobicity. The complete sequences of six cardiotoxin analogues, including one novel isoform never reported previously in this venom, were determined by N-terminal Edman degradation with the automatic pulsed-liquid phase sequencer and comparison of amino-acid compositions of purified toxin components. This novel cardiotoxin, a basic polypeptide of pI 10.13, comprises about 5% of total crude venom. It consists of 60 amino-acid residues with four disulfide bonds and shares about 80-92% sequence identity with several cytotoxins isolated from varied sources of different snakes. A systematic structure comparison of six major cardiotoxin isoforms was made by the secondary-structure predictions and hydropathy profiles based on their primary sequences. It is of interest to find that some subtle differences among these natural cardiotoxin analogues may lie in the intrinsic differences of the surface hydrophobicity of these isotoxins.