Kinetics of pulmonary angiotensin-converting enzyme and 5'-nucleotidase in vivo

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Abstract
Angiotensin-converting enzyme and 5′-nucleotidase line the luminal surface of pulmonary microvascular endothelium and participate in the synthesis and/or degradation of potent vasoactive substances. We applied Michaelis-Menten kinetics in simultaneous estimations of apparent constants Km and Amax (product of Vmax and microvascular plasma volume) of these two enzymes for the substrates 3H-labeled benzoyl-Phe-Ala-Pro and 14C-labeled 5′AMP, respectively, in vivo. Values of angiotensin-converting enzyme for benzoyl-Phe-Ala-Pro (Km = 10–11 microM; Amax = 12–13 mumol X min-1) were somewhat higher than published estimates in vitro and changed predictably in response to the known enzyme inhibitor captopril. Kinetic values of 5′nucleotidase for 5′AMP (Km = 3–4 microM; Amax = 3–4 mumol/min) were substantially lower than those reported in vitro but also responded predictably to the competitive inhibitor of 5′nucleotidase, adenosine 5′[alpha, beta-methylene]diphosphate. These data offer in vivo estimates of enzyme kinetics that are useful in revealing enzyme behavior in their normal physiological environment and provide means of evaluating the action of pharmacological, physiological, and pathological modulators of enzyme activity, in vivo.