Measurement of catalytic function of topo I in cultured fibroblasts. A serial dilution of topo I in the nuclear extracts obtained from SSc and control fibroblasts was used to relax 0.25 μg supercoiled DNA. A. The supercoiled DNA band is gradually diminished following increased amounts of topo I in the nuclear extracts in the relaxing assays. The efficiency of SSc topo I in relaxing the supercoiled DNA appeared to be less than that of control topo I in each concentration of nuclear extracts. B. Comparison of 11 paired SSc and control fibroblasts for mean values of intensity of supercoiled DNA bands after relaxing assay with different concentrations of topo I in the nuclear extracts. Each P-value of comparison at different dilution points is listed in the figure. Comparison of average band intensity of remaining supercoiled DNA in each of six dilutions between all SSc and all control fibroblasts showed a significant P-value (P = 0.0041) (Student's t-test). A = standard supercoiled DNA band; B = standard relaxed DNA bands; the numbers (1/32, 1/16, 1/8, 1/4, 1/2 and 1) indicate serial dilutions of topo I in nuclear extracts used for relaxing supercoiled DNA. The error bars indicate standard deviation (SD).