Abstract
Human plasma ACE concentration is ~4 nM, yet net ACE activity is <1 nM. Inhibition is evident on assaying ACE of increasing dilutions of plasma. Here, means are defined for computing net ACE activity of undiluted plasma, total ACE and % inhibition. Under first order kinetics, the Michaelis-Menten equation is v/s = Vmax/(Km (1 + i/Ki)), where i is inhibitor concentration; v/s is called apparent (app) (Vmax/Km). The inverted equation describes a straight line: app(Km/Vmax) = Km/VmaxKi(i) + Km/Vmax. When i, fractional concentration (fr conc) of plasma, is plotted against app(Km/Vmax), the y-intercept is true (Km/Vmax). The x-intercept is the fr conc of plasma for inhibition of ACE by half. Net ACE activity of undiluted plasma is computed for i = 1.0; 1/x is units of inhibitory activity. Plasmas of 37 healthy adults, 23 – 57 y.o., were assayed in fr conc of 0.2 – 0.00625 using [3H]hippuryl-His-Leu, 40 nM, in Hepes buffer, pH 8.0, plus NaCl and sodium sulfate, at 37oC. Results (mean±SD): Vmax/Km 0.3755±0.098/min, app(Vmax/Km) 0.0442±0.003/min, inhibitor units 7.29±2.0, inhibition 88.2±5.0%. Thus, <12% of plasma ACE is active. Inhibition, relieved by dialysis, is attributable to arginine, lysine, ornithine, cysteine, Cys-Gly and glutathione. Following a meal, their concentrations can double and increase inhibition, which raises the question, Is ACE activity physiologically modulated by food intake?
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