Evaluation of enzyme inhibition data in screening for new drugs

Enzyme inhibitors selected from either natural product screening or synthetic chemistry programmes can be characterized according to a number of criteria to determine their usefulness as potential drug candidates. Inhibition may be classified as either irreversible or reversible. Irreversible inhibitors may be described with respect to first order rate constants or half-times for inactivation. Reversible inhibitors should be evaluated with respect to Ki values rather than I50 values. Assays should be designed using the simplest and most precise procedures available. If possible, assays utilizing continuously-recording methods should be selected. When reversible inhibitors are desired, greater sensitivity for inhibition is achieved using low substrate concentrations. For irreversible inhibitors or tight-binding competitive inhibitors, low enzyme concentrations also result in greater inhibitory sensitivity. The order of addition of enzyme and substrate should be varied to determine whether inhibition requires preincubation of enzyme and inhibitor for maximum effect. Time-dependence of inhibition should also be established. Enzyme inhibitors should be specific for the enzyme that is being targeted. Novel competitive inhibitors of angiotensin-converting enzyme were isolated in the author's laboratory using the above philosophy of screening for enzyme inhibitors. The properties of the muraceins, phenacein and aspergillomarasmine A' are discussed.