The mechanisms that the body evolved over millions of years to metabolise foreign substances now enable it to meet the modern environmental challenges of tobacco smoke, hydrocarbon pollutants, insecticides and drugs. At times of high exposure, our enzyme systems respond by increasing in amount and so in activity, i.e. they are induced; when exposure falls off, enzyme production lessens. For example, a first alcoholic drink taken after a period of abstinence from alcohol may have quite a significant effect on behaviour but the same drink taken at the end of two weeks' regular imbibing may pass almost unnoticed because the individual's liver enzyme activity is increased (induced) so that alcohol is metabolised more rapidly and has less effect, i.e. tolerance has been acquired.
Inducing substances in general share some important properties: they tend to be lipid-soluble; they are substrates, though sometimes only minor ones, e.g. DDT, for the enzymes they induce and generally have long t!/2. The time for onset and offset of induction depends on the rate of enzyme turnover but significant induction generally occurs within a few days and it passes off over 2 or 3 weeks following withdrawal of the inducer.
It follows that the capacity of the body to metabolise drugs can be altered by certain medicinal drugs themselves and by other substances, especially when these are used long-term; clearly this phenomenon has implications for drug therapy. More than 200 substances have been shown to induce enzymes in animals but the list of proven enzyme inducers in man is much more restricted.
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