Interaction between drug and enzyme is in many respects similar to that between drug and receptor. Drugs may alter enzyme activity because they resemble a natural substrate and hence compete with it for the enzyme. For example, enalapril is effective in hypertension because it is structurally similar to that part of angiotensin I which is attacked by angiotensin-converting enzyme (ACE); by occupying the active site of the enzyme and so inhibiting its action enalapril prevents formation of the pressor angiotensin II. Carbidopa competes with levodopa for dopa decarboxylase and the benefit of this combination in Parkinson's disease is reduced metabolism of levodopa to dopamine in the blood (but not in the brain because carbidopa does not cross the blood-brain barrier). Ethanol prevents metabolism of methanol to its toxic metabolite, formic acid, by competing for occupancy of the enzyme alcohol dehydrogenase; this is the rationale for using ethanol in methanol poisoning. The above are examples of competitive (reversible) inhibition of enzyme activity.

Irreversible inhibition occurs with organophos-phorus insecticides and chemical warfare agents (see p. 437) which combine covalently with the active site of acetylcholinesterase; recovery of cholinesterase activity depends on the formation of new enzyme. Covalent binding of aspirin to cyclo-oxygenase

(COX) inhibits the enzyme in platelets for their entire lifespan because platelets have no system for synthesising new protein and this is why low doses of aspirin are sufficient for antiplatelet action.

Constipation Prescription

Constipation Prescription

Did you ever think feeling angry and irritable could be a symptom of constipation? A horrible fullness and pressing sharp pains against the bladders can’t help but affect your mood. Sometimes you just want everyone to leave you alone and sleep to escape the pain. It is virtually impossible to be constipated and keep a sunny disposition. Follow the steps in this guide to alleviate constipation and lead a happier healthy life.

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