The first mechanism-based inactivators for angiotensin-converting enzyme

A Method for Screening Enzyme Inhibitors Using Size Exclusion Chromatography and ESI-LC-MS/MS

A pilot study was performed for the development of a method to screen compound libraries using an electrospray mass spectrometer interfaced with liquid chromatography (LC). The mixture of compounds was obtained by combining low-molecular weight inhibitors of carboxypeptidase A (CPA), a representative zinc-containing proteolytic enzyme. After the incubation of the mixture with CPA, the enzyme-bound compounds were separated by size exclusion chromatography (SEC) from unbound compounds. The separation of compounds was affected by LC. Three compounds were identified, which represent the tight binding inhibitors of the library. These compounds were quantitated using an automatic switching valve to avoid the interference of buffer salts with the detection of analytes. The quantitated amounts of the compounds were found to be in good accordance with the Ki values. ( Journal of Biomolecular Screening 2005:30-35)

N-(Hydroxyaminocarbonyl)phenylalanine: a novel class of inhibitor for carboxypeptidase A

N-(Hydroxyaminocarbonyl)phenylalanine (1) was designed rationally as a new type of inhibitor for carboxypeptidase A (CPA). The designed inhibitor was readily prepared from phenylalnine benzyl ester in two steps and evaluated to find that rac-1 inhibits CPA in a competitive fashion with the Ki value of 2.09 microM. Surprisingly, inhibitor 1 having the D-configuration is more potent (Ki = 1.54 microM) than its antipode by about 3-fold. A possible explanation for the stereochemistry observed in the inhibition of CPA with 1 is presented.

Impact of mechanism-based enzyme inactivation on inhibitor potency: implications for rational drug discovery

Mechanism-based enzyme inactivators (MBEIs) have unique kinetic actions that make predictions of potency, selectivity, and potential for metabolic drug interactions more complex than for competitive antagonists. We have derived a mathematical relationship that links the influence of substrate concentration and binding constant ([S] and K(m), respectively), inhibitor concentration and binding constant ([I] and K(I), respectively), and inactivation rate constant (k(inact)) to enzyme activity (v) and maximal activity (V(max)) at any time (t). The kinetic behavior of this relationship was validated in murine-macrophage cell cultures using MBEIs of nitric oxide synthase (NOS). This initial equation was also used in the derivation of a new relationship that directly links the kinetic parameters of mechanism-based inactivation to inhibitory potency at a particular time (IC((t))(50)). Using this direct relationship, we observed that the predicted rank inhibitory potency of a series of MBEIs was improved over that predicted by the K(I) parameter alone. These relationships offer a fundamental understanding of the kinetics of MBEI action and may be useful in the evaluation of these compounds during the discovery process.

Comparison of methods for analyzing kinetic data from mechanism-based enzyme inactivation: application to nitric oxide synthase

The goals of this study were (1) to investigate the performance of 2 classical methods of kinetic analysis when applied to data from enzyme systems in which mechanism-based inactivation and enzyme degradation are present, and (2) to develop and validate a nonlinear method of kinetic data analysis that may perform better under these situations. A composite equation was derived to link various parameters that govern the kinetics of mechanism-based inactivation, viz., enzyme activity, inhibitor-binding affinity (K(I)), inactivation rate (k(inact)), and enzyme degradation (k(deg)). The relative accuracy and precision of parameter estimation by the Dixon and Kitz-Wilson methods and a new nonlinear method were evaluated by computer simulation. The behavior of these methods of analysis were validated experimentally, using the nitric oxide synthase enzyme, both in purified form and as expressed in murine macrophage cell cultures. We showed that the Dixon method, as expected, could not provide accurate estimates of K(I) in the presence of either enzyme inactivation or instability. The Kitz-Wilson method could provide accurate estimates of these parameters; however, the precisions of these estimates were poorer than those obtained using the nonlinear method of analysis. We conclude that the nonlinear approach is superior to classical methods of data analysis for enzyme inhibitor kinetics, based on better efficiency, accuracy, and precision.

Properties of analogues of an intermediate in the process of mechanism-based inactivation of carboxypeptidase A

Carboxypeptidase A (CPA), and other zinc-dependent proteases, facilitate an alpha deprotonation of judiciously designed ketones and amides. This adventitious reaction has been used in the development of effective mechanism-based inactivators for this family of enzymes. N-Acryloyl-L-phenylalanine, an intermediate in the process of mechanism-based inactivation of CPA by N-(3-chloropropionyl)-L-phenylalanine, was shown to be an affinity inactivator, but also a very poor substrate for the enzyme. Similarly, O-(acryloyl)-L-3-phenyllactate was shown to be both an affinity inactivator and a poor substrate for CPA. However, consistent with the trend established with other ester and amide substrates for CPA, O-(acryloyl)-L-3-phenyllactate is a better substrate than N-acryloyl-L-phenylalanine. N-(Propiolyl)-L-phenylalanine served only as a poor substrate for the enzyme. To gain insight into enzyme inactivation and the unexpected poor turnover of these molecules, molecular modeling of these compounds with the crystal structure of CPA was carried out. These analyses suggested that the smaller size of these molecules permits a binding mode which is somewhat different in the active site than with typical larger substrates, such that the transition-state species for hydrolysis is not greatly stabilized by the enzyme. The slow turnover of these species, along with their specific binding interactions with the enzyme active site have implications for the inactivation chemistry of CPA and other zinc proteases by this family of mechanism-based inactivators.