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  • Title: Measurements of serum ACE activity in vitro after administration of enalapril in man cannot reflect inhibition of the enzyme in vivo.
    Author: Weisser K, Schloos J.
    Journal: Methods Find Exp Clin Pharmacol; 1993; 15(6):413-8. PubMed ID: 8231461.
    Abstract:
    We previously demonstrated the dependency of serum angiotensin converting enzyme (ACE) inhibition by the inhibitor enalaprilat in vitro on the concentration of substrate and enzyme present in the assay, according to a competitive tight-binding mechanism. In the present study, the relevance of these findings for ex vivo measurements after administration of enalapril has been confirmed in serum samples of four patients which were assayed twice using different substrate concentrations in vitro. The measured extent of ACE inhibition in the samples was markedly different depending on the substrate concentration added in vitro (in relation to its Km value: S/Km, respectively), suggesting lower inhibition over time when higher S/Km was used. When the dilution factor and the added S/Km were taken into account the measured values could successfully be predicted from the respective serum enalaprilat concentrations by means of the concentration-effect model previously evaluated for the in vitro relationship (Emax tight model). Considering conditions which probably better reflect the situation in vivo (no dilution, physiological substrate concentrations far below Km) we simulated a time course of in vivo serum ACE activity in these patients which suggests almost complete inhibition of serum ACE over time in contrast to the in vitro measurement. Thus, we conclude that the usual ex vivo measurements of ACE activity lead to an underestimation of the extent of inhibition because of sample dilution and high exogenous substrate in vitro, and therefore must fail to reflect enzyme inhibition in vivo.
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