Thiol isomerase activity of human ERP5. Thiol isomerase activity was assessed as the ability to refold denatured, scrambled RNAse and enhance the degradation of cyclic 2′,3′-cytidine monophosphate, as followed by ultraviolet/visible (UV/vis) spectroscopy. (A) Activity was observed for a recombinant ERP5 fusion protein (21 μg/mL) and a recombinant PDI fusion protein (11 μg/mL) relative to samples containing only cyclic 2′,3′-cytidine monophosphate (blank) or only fusion protein. (B) Anti-ERP5 polyclonal antibodies (24 μg/mL) raised in sheep were able to partially inhibit the thiol isomerase activity of a recombinant ERP5 fusion protein (30 μg/mL). Preimmune IgG (24 μg/mL) and monoclonal anti-PDI antibodies (28 μg/mL) were found to possess no such inhibitory activity. Data are presented as mean ± SE from 3 different determinations. *P < .05.