Consecuencias derivadas de la postura interpretativa asumida

Enzymes are true catalysts—they are not changed appreciably during a reaction, but as proteins they are subject to decomposition in the body. Under normal conditions the rate of enzyme production equals its rate of degradation. Obviously, however, any abnormal condition, such as malnutrition or disease, that decreases the overall rate of protein synthesis may also result in decreased availability of enzymes. Since the liver plays such an important role in protein synthesis as well as in metabolism, malfunction of this organ may reduce the rate of drug biotransformation. In addition, there are many therapeutic agents, dietary components, and environmental chemicals that interact with enzymes of biotransformation and inhibit their activity. Examples include the antiulcer agent

omeprazole, the antifungal agent ketoconazole, and a component of grapefruit juice, all documented CYP450 inhibitors.

To catalyze a reaction, an enzyme must be able to combine with its substrate.

Therefore, any agent that interferes with a substrate’s access to active binding sites will also decrease the rate of metabolism, even when the concentration of enzyme is normal.

A decrease in metabolic rate of a given substrate is said to be competitive inhibition when the interfering agent is (1) a compound that is itself a substrate for the enzyme or (2) a compound that undergoes no catalytic change but combines reversibly with the active sites of the enzyme because of its structural similarity to the substrate. Methacholine is an example of a substrate that competes with acetylcholine for the active sites on the enzyme cholinesterase (cf. p. 154). The second type of competitive inhibition is exemplified by the action of amphetamine. As we have seen, amphetamine is not biotransformed by monoamine oxidase, but it can inhibit the metabolism of tyramine, a natural substrate of this enzyme. These competitive interactions may be represented as follows:

(Equation 3)

(Equation 4)

where E is enzyme, S1 and S2 are substrates, I is a nonsubstrate and P1 and P2 are the end products of the metabolism of S1 and S2, respectively.

In Equation 3, S1 may represent acetylcholine, the natural substrate of cholinesterase (E), and S2 may be the drug substrate, methacholine, which can bind to the same active sites on the enzyme as acetylcholine. In Equation 4, S1 may be tyramine, the endogenous substrate of monoamine oxidase, and the nonsubstrate, I, may be amphetamine, which combines with the active sites of this enzyme even though this combination does not lead to biotransformation of the drug.

In both cases, the effect of two agents competing for the same quantity of enzyme is to reduce the rate of metabolism of the primary substrate, S1, as predicted by the law of mass action. First, the degree of inhibition produced is dependent on the concentration of the primary substrate, S1, relative to the concentrations of either S2 or I. The greater the number of molecules of S1 present in the total number of molecules capable of combining with active sites, the greater the possibility that a molecule of S1 will complete the binding reaction. Then, since the rate of metabolism is proportional to substrate concentration, increasing the ratio of S1 to either S2 or I will decrease inhibition. A sufficiently high ratio of S1 to either S2 or I will force almost complete occupancy of active sites by S1 despite the presence of either S2 or I (Fig. 8–8). Thus competitive inhibition can be overcome by a large enough concentration of substrate. Of course the extent of inhibition is also determined by the relative rates of dissociation of the complexes formed, i.e. on the rates of dissociation of ES1, ES2 or EI. The rates of dissociation of all three complexes must be sufficiently rapid to make free, unoccupied sites available for continuous combination with molecules of S1.

Figure 8–8. Effect of inhibitors on enzyme activity. The rate of the catalyzed reaction is plotted as a function of substrate concentration when the amount of enzyme is constant and the concentration of each of the inhibitors is also constant.

Inhibition of metabolism may also be brought about by an agent unrelated in structure to the substrate but capable of combining with the enzyme in such a way as to prevent the formation of an enzyme-substrate complex. This is termed noncompetitive inhibition.

Many heavy metals, such as mercury, lead or arsenic, and the organic phosphate insecticides are typical noncompetitive inhibitors. Since noncompetitive inhibitors do not combine with the enzyme in the same manner as the substrate, an excess of substrate cannot displace the inhibitor from the enzyme surface. Noncompetitive inhibition may be reversible or irreversible; the important point is that the concentration of substrate does not influence the reversibility or the degree of inhibition (Fig. 8–8). When the action of a noncompetitive inhibitor is irreversible, the enzymatic activity is destroyed and new molecules of enzyme must be synthesized before full enzymatic activity is restored. For example, in the blood, aspirin inactivates cyclooxygenase irreversibly in platelets to prevent clotting; since the life of a platelet is only a few days, new platelets must be synthesized to restore cyclooxygenase activity. Chemicals that are substrates of an enzyme but reduce its activity irreversibly are referred to as suicide inhibitors.

The inhibition of enzyme activity has pharmacologic significance aside from decreasing the rate of drug biotransformation and prolonging the duration of drug action.

In certain pathologic conditions, the inhibition of specific normal metabolic processes may be beneficial. Under these circumstances, a chemical that can inhibit the appropriate enzyme system is a useful therapeutic agent; its receptor is the enzyme and not an integral, macromolecular component of tissue. Enzyme inhibitors that are therapeutically useful drugs include inhibitors of cholinesterase, monoamine oxidase, carbonic anhydrase, and xanthine oxidase, mentioned previously in this text. There are many

others as well, such as the aspirin family of drugs that inhibit the fatty-acid oxidizing enzyme cyclooxygenase (COX) and the antihypertensive ACE inhibitors that block the generation of a vasoconstrictor peptide by angiotensin-converting enzyme (ACE).