Analyses of characterization and structure of NO synthase and NO-carrier protein from blood-sucking insects

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Rhodnius prolixus is a blood-sucking Hemipteran insect which is known as a vector of American trypanosomiasis. This insect has several physiologically active molecules in its saliva that assist its blood-sucking behavior. We have already reported purification and the cDNA sequence of a salivary anticoagulant, prolixin-S, from this insect and demonstrated that prolixin-S was an inhibitor of the intrinsic Xase. We also reported that prolixin-S bound nitric oxide (NO) in its heme group as a vasodilator and a platelet aggregation inhibitor.We further studied on its inhibitory mechanisms of the blood coagulation. We found prolixin-S specifically bound to factor IX/IXa in the presence of CaィイD12+ィエD1 ions. Light scattering and surface plasmon resonance (SPR) studies showed that prolixin-S interfered with factor Ix/IXa binding to the phospholipid membrane, indicating that prolixin-S inhibit Xase activity of factor IXa by interference with its Xase complex formation. Furthermore, reconstitution experiments showed that prolixin-S binding to factor IX strongly inhibited factor IXa generation by factor XIa. We also found that prolixin-S inhibited factor IXa generation by factor VIIa/tissue factor complex and factor IXalfa generation by factor Xa. These results suggests that prolixin-S inhibits both intrinsic and extrinsic coagulations by sequential inhibition of all coagulation pathways in which factor IX participates. It was also suggested that prolixin-S may bind to factor IX/IXa by recognizing conformational change of the Gla domain induced by CaィイD12+ィエD1 binding. To gain further understanding, we are now trying to locate the interactive sites using synthesized Gla-residue peptide.