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Lung angiotensin-converting enzyme kinetics from indicator-dilution and constant-infusion methods
J H Linehan1, R D Bongard, D L Roerig
1Department of Biomedical Engineering, Marquette University, Milwaukee, Wisconsin 53233.
The American Journal of Physiology
|February 1, 1990
Summary
Three methods for evaluating angiotensin-converting enzyme kinetics in rabbit lungs yielded similar results for Vmax and Km. Theoretical differences did not significantly impact quantitative outcomes, allowing method selection based on practical considerations.
Area of Science:
- Pharmacology
- Enzymology
- Physiology
Background:
- Angiotensin-converting enzyme (ACE) plays a crucial role in cardiovascular regulation.
- Evaluating ACE kinetics is essential for understanding its physiological function and drug interactions.
- Benzoyl-phenylalanyl-alanyl-proline (BPAP) is a substrate used to study ACE activity.
Purpose of the Study:
- To compare the kinetic parameters (Vmax and Km) of BPAP hydrolysis by ACE using three distinct evaluation methods.
- To assess the impact of theoretical assumptions, including capillary transit time heterogeneity and bolus dispersion, on kinetic measurements.
- To determine if theoretical differences between methods lead to significant quantitative variations in Vmax and Km.
Main Methods:
- Method A: Utilized arteriovenous concentration differences during constant BPAP infusion.
- Methods B1 & B2: Employed trace doses of [3H]BPAP during constant unlabeled BPAP infusion to estimate kinetic parameters.
- Method C: Used boluses of unlabeled BPAP with [3H]BPAP tracer to assess kinetics across a range of concentrations.
Main Results:
- All three methods provided data to calculate Vmax and Km, assuming Michaelis-Menten kinetics.
- Mean Vmax values ranged from 180 to 217 nmol/s, and mean Km values ranged from 7.8 to 10.4 microM across methods.
- Statistical analysis revealed no significant differences in Vmax or Km values between the evaluated methods.
Conclusions:
- Theoretical differences in the mathematical models underlying the kinetic evaluation methods did not introduce significant quantitative discrepancies.
- Random errors inherent in each method appeared to be a more substantial factor than theoretical model variations.
- The choice of method for evaluating ACE kinetics in isolated rabbit lungs can be based on practical considerations rather than significant differences in kinetic outcome.