Angiotensin I Converting Enzyme of the Lung
Angiotensin I converting enzyme or kininase II (DH) was purified from hog lung. The preparation was homogeneous in disc gel electrophoresis. Lung DH inactivated bradykinin and converted angiotensin I to angiotensin II. DH cleaved dipeptides in vitro from the C-terminal end of various substrates, including angiotensin I, bradykinin, B-chain of insulin, and shorter peptides, which were employed as substrates in spectrophotometric experiments. Several peptides, including glutathione, insulin, and the synthetic peptides SQ 20881 and peptide C, inhibited both plasma and lung DH. The latter inhibitors were inactive against carboxypeptidase N or kininase I. A sensitive and accurate assay of DH was developed, using 14C-DNS-Gly-Gly-Gly, a radioactive and fluorescent substrate. DH was coupled to Sepharose-4B to form a waterinsoluble complex. This insoluble enzyme cleaved both bradykinin and angiotensin I as determined by bioassay or radioimmunoassay. DH was also studied in the rat lung perfused in situ with (14C-Leu10)-angiotensin I and 14C-DNS-Gly-Gly-Gly substrates. During passage through the pulmonary circulation, half of the angiotensin I was converted to angiotensin II by the liberation of His-Leu; Gly-Gly was cleaved from the dansyl substrate. The activity of DH in the perfused lung of hypertensive rats was normal. Inhibitors such as insulin or SQ 20881 also blocked the action of DH in lung perfusion experiments.