Serum angiotensin-converting enzyme. Isolation and relationship to the pulmonary enzyme.

Angiotensin-converting enzyme from rabbit serum was purified almost 60,000-fold to apparent homogeneity by a procedure exploiting its affinity for antibodies prepared against the enzyme from lung. The pure serum and pulmonary enzymes exhibited identical behavior during gel filtration, sucrose gradient centrifugation, and disc gel electrophoresis in the reduced, denatured state. Their catalytic properties with hippurylhistidylleucine, angiotensin I, and bradykinin as substrates were similar and their reactivity with antilung enzyme antibody was indistinguishable as examined by immunodiffusion, inhibition dose-response curves, and radioimmunoassay. Their content of fucose, mannose, galactose, and N-acetylglucosamine was also comparable; however, N-acetylneuraminic acid was much more abundant in the serum glycoprotein. This difference may reflect selective removal of sialic acid-deficient enzyme molecules from the circulation by the hepatic lectin which has been postulated to initiate the catabolic phase for plasma glycoproteins (Ashwell, G., and Morell, A.G. (1974) Adv. Enzymol. Relat. Areas Mol. Biol. 41, 91-128).