Toxicity to crustacea due to polypeptide-phospholipase interaction in the venom of a chactoid scorpion

The toxic factors to isopods (crustacea) were isolated from the venom of the chactoid scorpion Scorpio maurus palmatus (Scorpionidae) by the aid of column chromatography, and their purity was assessed by disc electrophoresis, analytical ultracentrifugation, isoelectrofocusing, and amino acid analysis. The toxicity to isopods is attributed to two groups of components: (a) Low-molecular-weight basic polypeptides possessing about 3 and 8% of the crude venom lethality and paralytic potency to isopods, respectively. These components are characterized by very similar and unique amino acid compositions of 31 to 34 amino acids with molecular mass of about 3.5 kDa and a deficiency in methionine, leucine, phenylalanine, histidine, and tryptophan. (b) Toxic phospholipases are also toxic to insect but not to mammals. A lethal phospholipase which contained 37% of the total venom phospholipase activity and 11% of its toxicity to isopods was purified. This phospholipase consists of 125 amino acids (Mr 14,581) and is a hydrophobic, acidic protein composed of two isoenzymes (pI 4.7 and 4.9). This enzyme demonstrates an A2-type positional specificity (EC 3.1.1.4) with pH and temperature optima of 7.5-8.0 and 40-50 degrees C, respectively, and high calcium requirements. The lethal potency of the basic polypeptides is evidently increased by the addition of low, sublethal doses of the pure phospholipase. Such synergism was not observed with regards to their paralytic activity. The pharmacological significance of these data is discussed.