Abstract OBJECTIVE To investigate the specific binding of nucleic acid aptamers(k-α2ct) with plasminogen Kringle 5(K5) on the function of K5 in inhibiting proliferation and migration of vascular endothelial cells and promoting their apoptosis. METHODS The cloning and expression of recombinant K5 protein were performed by using a prokaryotic system, and the isolation and purification of the expressed K5 protein were performed by affinity chromatography. The affinity and specificity of K-a2ct and K5 were verified using isothermal titration calorimetry(ITC) and enzyme-linked oligonucleotide adsorption assay(ELONA). The effect of K-a2ct on the function of K5 in inhibiting the proliferation and migration of human umbilical vein endothelial cells(HUVEC) was investigated by CCK-8 and cell scratch assay. The apoptotic morphology of HUVEC cells stained with Hoechst 33342 was observed by laser confocal microscopy, and the effect of K-a2ct on the apoptosis- promoting function of K5 in HUVEC cells was also examined by Annexin V/PI double-stained flow cytometry. RESULTS Recombinant protein K5 was efficiently expressed in Escherichia coli and purified by affinity chromatography, identified as having a relative molecular weight of 12 kDa and a concentration of 0.32 mg·mL-1. ITC and ELONA results demonstrated that K-a2ct had a strong affinity and good selectivity for K5, showing the affinity specificity of a typical nucleic acid aptamer. CCK-8 and cell scratching assays showed that K-a2ct could inhibit the anti-proliferation and anti-migration effects of K5 on HUVEC cells in a dose-dependent manner. The laser confocal and flow cytometry results showed that K-a2ct inhibited the apoptosis-promoting function of K5 on HUVEC cells, mainly affecting late apoptosis of HUVEC cells effected by K5 but having little effect on early apoptosis. CONCLUSION The nucleic acid aptamers K-a2ct binds to K5 with high affinity and specificity, and inhibits its anti angiogenic function in a dose-dependent manner. It has great potential in targeting the regulation of K5 concentration and function in vivo and promoting angiogenesis.