Inhibition of ERp5 disulfide reductase and isomerase activity by anti-ERp5 antibody. (A) ERp5 disulfide reductase activity was measured by reduction of di-E-GSSG as a function of time. The inhibition of ERp5-catalyzed (50 nM) reduction of di-E-GSSG was determined in the presence of increasing amounts of anti-ERp5 antibody. His-PDI (50 nM) reductase activity (B) or His-ERp57 (100 nM) reductase activity (C) was measured in the di-E-GSSG assay in the presence of anti-ERp5 antibody. For panels A-C, anti-ERp5 antibody: none (□), 0.3 µM (▲), 0.6 µM (●), and 2.2 µM (○); preimmune IgG: 2.2 µM (▪); DTT, no enzyme: (△). N = 3 in triplicate. (D) The isomerase activity of ERp5 (1 μM) was measured in the RNase renaturation assay. The inhibition of isomerase activity was measured in the presence of increasing concentrations of anti-ERp5 antibody. (E) The isomerase activity of PDI (1 µM) was measured in the RNase renaturation assay in the absence and presence of anti-ERp5 antibody. (F) The isomerase activity of ERp57 (2.5 µM) was measured in the RNase renaturation assay in the absence and presence of anti-ERp5 antibody. For panels D-F, anti-ERp5 antibody: none (□), 0.3 µM (▲), 0.6 µM (●), and 2.2 µM (○); preimmune IgG: 2.2 µM (▪); denatured and reduced RNase: (△). N = 3 in triplicate. Abs, absorbance.