Studies on the mechanism of the Arg327Ile mutant causing defect in the function of factor Ⅸ / 中华检验医学杂志

ABSTRACT

Objective To investigate function of Arg327Ile (R327I) and Arg327Ala(R327A) FⅨ mutants and to study the molecular pathogenesis of haemophilia B(HB) caused by R3271 mutation.Methods Hygromycin-resistant cell line was screened and the secretion of FⅨ antigen into the medium was measured by ELISA.The cell line with appropriate expression levels of F Ⅸ antigen was selected for culture.Recombinant F Ⅸ (rF Ⅸ ) was purified from concentrated medium by two step methods of Q-Sepharose Fast Flow and anion exchange chromatography.The concentration and purity of rF Ⅸ were determined by ELISA and SDS-PAGE,respectively.The activation of wild-type ( WT),R327I and R327A of rFⅨ by FⅦa/TF/Ca2+ or FⅪa/Ca2+ was identified by Western blot in different time periods.The FⅨa and FⅧa complex formed by interaction with different concentrations of FⅧa was used to activate F X,the apparent dissociation constant (Kd) for FⅧa binding was calculated by the kinetic results.The kinetic data of the activation of FX by WT,R327I and R327A FⅨa with or without FⅧa were calculated.Results The amount of WT,R327I and R327A rFⅨ were 450,210,64 μg,and the purity of rFⅨ was confirmed by SDSPAGE.Both R3271 and R327A could be normally activated by FⅧa/TF/Ca2+ or FⅪa/Ca2+.Kd for FⅧa binding showed that the binding capacities of R327I and R327A were 4 and 5 times lower than WT,respectively.The catalytic efficiencies of R327I and R327A F Ⅸ a for F X were 6 and 8 times lower with FⅧa,and 3 and 7.4 times lower without F Ⅷ a,respectively.Conclusions R327I and R327A rF Ⅸ mutants impair their binding to the FⅧa.The site on R327 contributes to FⅧa binding.It is partly related to the activation of FX.The low FⅧa binding to R327I FⅨa may cause HB.