Domain structures of porcine lung and testicular angiotensin-converting enzymes (ACE) were studied by radiation inactivation to test the hypothesis that lung ACE has two catalytic sites localized to discrete, structurally independent domains (the N- and C-domains) of approximately equal size. The minimum functional sizes of lung and testicular ACE, calculated from the inactivation curves obtained, were 140 and 74 kDa, respectively. Since testicular ACE has been demonstrated to contain only the C domain, this result indicates that the two domains in lung ACE are not independent but are, in fact, structurally tightly linked.
