Phospholipases A(2) from Callosellasma rhodostoma venom gland - Cloning and sequencing of 10 of the cDNAs, three-dimensional modelling and chemical modification of the major isozyme

Callosellasma rhodostoma (Malayan pitviper) is a monotypic Asian pitviper o f medical importance. Three acidic phospholipases A(2) (PLA(2)s) and one ba sic PLA(2)-homolog were purified from its venom while 10 cDNAs encoding dis tinct PLA(2)s were cloned from venom glands of a Thailand specimen of this species. Complete amino-acid sequences of the purified PLA(2)s were success fully deduced from their cDNA sequences. Among the six un-translated PLA(2) cDNAs, two apparently result from recombination of its Lys49-PLA(2) gene w ith its Asp49-PLA(2) genes. The acidic PLA(2)s inhibit platelet-aggregation , while the noncatalytic PLA(2)-homolog induces local edema. This basic PLA (2)-homolog contains both Asp49 and other, unusual substitutions unique for the venom Lys49-PLA(2) subtype (e.g. Leu5, Trp6, Asn28 and Arg34). Three-d imensional modelling of the basic protein revealed a heparin-binding region , and an abnormal calcium-binding pocket, which may explain its low catalyt ic activity. Oxidation of up to six of its Met residues or coinjection with heparin reduced its edema-inducing activity but methylation of its active site His48 did not. The distinct Arg/Lys-rich and Met-rich region at positi ons 10-36 of the PLA(2) homolog presumably are involved in its heparin-bind ing and the cell membrane-interference leading to edema and myotoxicity.