Fig. 5. Bioactive protein delivery with high serum-resistant performance. (a) Mechanisms of substrates catalyzed into products by β-gal. (b) Photographs of β-gal transfection efficacy through X-gal staining assay in 143B cells. (c) ONPG assay for determining β-gal activity after intracellular delivery by ND polymers. Cell lysate that added the same amount of β-gal was tested as 100% enzyme activity. (d) β-Gal enzyme activity of 143B cells, which were stained with resorufin-β-d-gal. (e) Quantitative analysis of resorufin-β-d-gal staining assay via a standard curve of different β-gal concentrations. (f) Viability of 143B cells treated with ND/β-gal complexes for 24 h, followed by 5-flu-O-β-gal treatment for another 24 h. Data are shown as mean ± s.d. (n = 3). ^N.S.p > 0.05 and *p < 0.05 were calculated by Student's t-test. Free β-gal was a negative control. PULSin and TransEx were used as positive controls.