In the present study, one-step purification of angiotensin-converting enzyme (ACE, peptidyldipeptidase A, EC 188.8.131.52), responsible for regulation of blood pressure, was achieved using affinity chromatography from human plasma. The enzyme was purified 12,860-fold with a specific activtiy of 5080 EU/mg protein. Optimum temperature and pH were determined for the enzyme as 35-40 degrees C and pH7.4-7.5, respectively. The purity of ACE was determined by SDS-PAGE and the enzyme showed two bands at 60 and 70kDa on the gel. The native molecular weight of ACE was found to be 260kDa by gel filtration chromatography, demonstrating that the enzyme has a heterodimeric structure. Natural fatty acids of Nigella sativa (Ranunculaceae) were isolated by means of column chromatography. The structures of these compounds were determined using NMR and GC-MS. The results showed that high concentrations of linoleic, oleic and palmitic acids were isolated from the plant. The effect of six fractions (Fr 1-6) on ACE activity was examined. Fraction 3 increased the ACE activity while the other fractions decreased the enzyme activity. The concentrations of the fractions inhibiting the half-maximum activity of the enzyme were calculated as 1.597mg/mL for Fr 1, 0.053mg/mL for Fr 2, 0.527mg/mL for Fr 4, 0.044mg/mL for Fr 5 and 0.136mg/mL for Fr 6 using a Lineweaver-Burk graph.