Factor Xa (FXa) inhibitor from the nymphs of the camel tick Hyalomma dromedarii

An inhibitor of factor Xa (FXa) was isolated from the nymphs of the camel t ick Hyalomma dromedarii by a combination of chromatography on DEAE-cellulos e and Sephacryl S-300 columns. The isolated nymphal FXa inhibitor turned ou t to be a homogenous preparation of a single polypeptide chain (15 kDa) as judged by both the native and denatured SDS-PAGE. Its pI value ranged from 7.7 to 7.9. The inhibitor is a potent anticoagulant since it prolonged both the activated partial thromboplastin time (APTT) and the prothrombin time (PT) of the camel plasma in a concentration-dependent manner. Its activity was threefold lower toward thrombin than FXa, but it did not inhibit any of the proteases; trypsin, alpha -chymotrypsin, papain, pepsin and subtilisin . The inhibitor binds at two sites on FXa uncompetitively with an inhibitio n constant (K-1) value of 134 nM. (C) 2001 Elsevier Science Inc. All rights reserved.