This work reports the purification and biochemical characterization of angiotensin I-converting enzyme (ACE) from ostrich (Struthio camelus) lung. The molecular weight of the purified enzyme was approximately evaluated to be 200 kDa and the maximum enzyme activity was observed at pH 7.5. The enzyme activity was increased by detergents of Triton X-100 (0.01%), cetyltrimethylammonium bromide (CTAB) (0.1 and 1 mM) and sodium dodecyl sulfate (SDS) (0.1 mM), while decreased by Triton X-100 (1% and 10%) and SDS (I mM and 10 mM). The secondary and tertiary structure and activity of ACE in the absence and presence of trifluoroethanol (TFE) were investigated using circular dichroism, fluorescence quenching and UV-visible spectroscopy, respectively. Our results revealed that TFE stabilizes ACE at low concentrations, while acts as a denaturant at higher concentration (20%). The Km, Kw and Kcai/Km values of ostrich ACE towards FAPGG were 0.8 x 10(-4)M, 59,240 min(-1) and 74 x 10(7) min(-1) M-1, respectively. The values of IC50 and K-i for captopril were determined to be 36.5 nM and 16.6 nM, respectively. In conclusion, ostrich lung ACE is a new enzyme which could be employed as a candidate for studying ACE structure and its natural or synthetic inhibitors. (C) 2013 Elsevier Ltd. All rights reserved.