Isolation and properties of the angiotensin-converting enzyme from human lungs

Using chromatofocusing, an angiotensin-converting enzyme (EC 3.4.15.1) has been isolated from human lung. The procedure allows for 24 300-fold purification of the enzyme. The enzyme specific activity is 36.3 u. per mg protein; Mr as determined by polyacrylamide gel electrophoresis is 150 000. The lung enzyme after solubilization by trypsin treatment was found to be heterogeneous. Four isoforms of the enzyme with pI 5.3, 4.9, 4.8 and 4.6 were identified. The pH-optimum for the enzyme with respect to hippuryl-L-histidyl-L-leucine hydrolysis lies at 8.3; Km = 2.8 mM. The effect of Cl- on the enzyme activity was studied. It was found that the bradykinin-potentiating factor (SQ 20 881) inhibits the human lung angiotensin-converting enzyme (I50 = 1.6 X 10(-8) M).