Plasmalemmal metabolic activities in cultured calf pulmonary arterial endothelial cells

We have determined kinetic characteristics of angiotensin converting enzyme, 5'-nucleotidase and transmembrane serotonin uptake and metabolism in cultured calf pulmonary arterial endothelial cells. Angiotensin converting enzyme activity was 2.8 +/- 0.03 Units/10(6) cells (N = 19; 1 Unit: amount of enzyme required to metabolize 1% of substrate, benzoyl-Phe-Ala-Pro, in 1 min under conditions of first order reaction kinetics) in confluent monolayers and 2.31 +/- 0.06 Units/10(6) cells (N = 20) in homogenates. 5'-Nucleotidase activity (substrate: 5'-AMP) was 0.25 +/- 0.01 Units/10(6) cells (N = 19) in monolayers and 0.26 +/- 0.01 Units/10(6) cells (N = 20) in homogenates. Kinetic constants for angiotensin converting enzyme were: Km = 7.6 microM, Vmax = 5.2 nmol/hour/10(6) cells and for 5'-nucleotidase: Km = 52.6 microM, Vmax = 6.3 nmol/hour/10(6) cells. These data confirm that both angiotensin converting enzyme and 5'-nucleotidase are ectoenzymes with no cytoplasmic activity. Serotonin uptake exhibited both a saturable (Km = 0.27 microM, Vmax = 17 pmol/hour/10(6) cells) and a non-saturable component.