Putative extrinsic blood coagulation pathway inhibitors from the tick Ornithodoros savignyi

Abstract

Commercial (high-grade) BaS04 selectively adsorbs two proteins from crude 0. savignyi salivary gland extracts. They co-purify during reversed-phase HPLC, but can be separated by hydrophobic-interaction chromatography. Both proteins have been characterized in terms of their molecular mass, amino acid composition and one partial internal amino acid sequence was determined. Their molecular masses were established through electro-spray mass spectrometry as 9333 Da and 9173 Da, respectively. The 9.3 kDa protein was designated BSAP1 and the 9.1 kDa protein BSAP2. Their amino acid compositions shows significant differences, in particular the presence of 6-7 and 8 cysteine residues in BSAP1 and BSAP2, respectively. It is therefore unlikely that these proteins are isoforms. All of the cysteine residues are involved in the formation of disulphide bonds, the only possible exception being one residue in BSAP1. Both proteins appear to be N-terminally blocked. An internal amino acid sequence Asp/Ser-Gly-Gly-Xxx-Xxx-Ile-Leu-Gly was obtained by sequencing a fragment of the cyanogen bromide cleaved BSAP2. It was suspected that these proteins might exhibit anticoagulant activity. The prothrombin time (PT) and activated partial thromboplastin time (aPPT) in the presence of the presumptive inhibitors were therefore evaluated. The aPPT was not significantly prolonged. The PT however did indicate a slight delay in the clotting time. This delay is not due to inhibition of factor VII, one of only two unique coagulation factors in the extrinsic pathway. The other factor is thromboplastin, also known as tissue factor. The nature of the protein adsorption to BaS04 was examined. From literature it is known that ϒ-carboxyglutamic acid-containing proteins, as well as some hydroxyproline and hydroxylysine-rich glycoproteins adsorb selectively to BaS04. The BSAPs were analysed for the presence of these modified amino acids, but all tests proved negative. The absence of Gla residues was determined using a Gla-specific stain on a polyacrylamide gel and was confirmed by performing mass spectrometry on native and decarboxylated protein samples. The absence of hydroxyproline and hydroxylysine was demonstrated by amino acid analysis. Both BSAPI and BSAP2 bind to neutral and negative membranes. BSAPI binds neutral and negative membranes more strongly than BSAP2. Its affinity for negative membranes is however much lower than its affinity for neutral membranes. In contrast, BSAP2 binds both membranes equally strongly. The binding of the proteins to the membranes was significantly lowered upon pre-incubation with Ca2+.