Enzyme Induction And Inhibition

Most biological systems, and especially humans, are exposed to a large number of different chemicals in the environment. Thus, pesticides, natural contaminants in food, industrial chemicals and agricultural pollutants may all contaminate the environment and thereby affect various biological systems in that environment. Humans and certain other animals may also be exposed to drugs and food additives, and humans are exposed to substances in the workplace. These chemicals may modify their own disposition and that of other chemicals in several ways. One way this may occur is by an effect on the enzymes involved in the metabolism of foreign compounds; these enzymes may be induced or inhibited. By altering the routes or rates of metabolism of a foreign compound, either induction or inhibition clearly can have profound effects on the biological activity of the compound in question.

Enzyme induction

Although first reported with the microsomal mono-oxygenases, it is now known that a number of the enzymes involved in the metabolism of foreign compounds are inducible. Thus, as well as the cytochromes P-450, NADPH cytochrome P-450 reductase, cytochrome b5, glucuronosyl transferases, epoxide hydrolases and glutathione transferases are also induced to various degrees. However, this discussion concentrates on the induction of the mono-oxygenases with mention of other enzymes where appropriate.

The induction of the microsomal enzymes has been demonstrated in many different species including humans, and in various different tissues as well as the liver. Induction usually results from repeated or chronic exposure although the extent of exposure is variable. The result of induction is an increase in the amount of an enzyme; induction requires de novo protein synthesis, and therefore an increase in the apparent metabolic activity of a tissue in vitro or animal in vivo. Consequently, inhibitors of protein synthesis, such as cycloheximide, inhibit induction. It is a reversible, cellular response to exposure to a substance. Thus, it can be shown in isolated cells, such as hamster foetal cells in culture, that exposure to benzo[a]anthracene induces aryl hydrocarbon hydroxylase activity (AHH), one of the isoenzymes of cytochrome P-450.

A large variety of substances have been shown to be inducers, probably numbering several hundred. Apart from the fact that many are lipophilic and organic, there are no common factors and many chemical classes of compound are included (table 5.19). However, there are several types of induction which can be differentiated, and within some of these types inducers show certain structural similarities. For example, inducers of the polycyclic hydrocarbon type tend to be planar molecules. Planar and non-planar polychlorinated biphenyls (figure 5.30) differ in the type of induction they will cause. Thus, 3, 3', 4, 4', 5, 5/-hexachlorobiphenyl is a planar molecule which is an inducer of the polycyclic hydrocarbon type. 2, 2', 4, 4', 6, 6'-Hexachlorobiphenyl is a non-planar molecule due to the steric hindrance between the chlorine atoms in the 2- and 6-positions, and is a phenobarbital type of inducer. The variety and type of inducing agents are shown in table 5.19. Some compounds may indeed be mixed types of inducers, and thus mixtures of planar and

TABLE 5.19 Major types of cytochrome P-450 enzyme inducing agent



Isoenzymes induced



CYP2B1/2; 2C






TCDD; benzo[a]pyrene




Acetone; isoniazid;



Pregnenolone 16a-carbo


nitrile; dexamethazone




Ciprofibrate; Nafenopin

TCDD: 2, 3, 7, 8-tetrachlordibenzo-p-dioxin.

TCDD: 2, 3, 7, 8-tetrachlordibenzo-p-dioxin.

FIGURE 5.30 The structures of planar (A) and non-planar (B) hexachlorobiphenyls.

3 ¿'.S.ff-Haxfltfrforoa ptwnyl ^«.F-H&iacfiicrauprLenji

FIGURE 5.30 The structures of planar (A) and non-planar (B) hexachlorobiphenyls.

non-planar polychlorinated biphenyls are found to act as inducers of both the polycyclic and phenobarbital type.

As already discussed in Chapter 4, cytochrome P-450 has many forms or isoenzymes which differ in their ability to catalyse particular reactions. Some of these forms of cytochrome P-450 are found in normal liver tissue and are 'constitutive', whereas others are only apparent after induction. Constitutive as well as non-constitutive forms of cytochrome P-450 are inducible. Some of the major forms of cytochrome P-450 which are induced are shown in table 5.19. It should be noted, however, that this is not an exhaustive list and there are species and tissue differences in the constitutive and induced forms of cytochrome P-450.

Thus, induction can change the proportions of isoenzymes in a particular tissue, and may increase the activity of a normally insignificant form by many times. Although phenobarbital induction increases the overall concentration of cytochromes P-450 in the liver by about three-fold, specific isoenzymes may be increased up to 70-fold. Treatment with 3-methylcholanthrene can increase a specific form of the enzyme by a similar order.

Induction of the microsomal enzymes may also have other effects as well as the increased production of particular enzymes and isozymes. Here again the different types of inducer vary. Thus, the barbiturate type of inducer differs significantly from the polycyclic hydrocarbon type as can be seen from the list below.

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Some of the characteristic changes caused by the barbiturate type of inducer are:

1 increase in smooth endoplasmic reticulum

2 increase in liver blood flow

3 increase in bile flow

4 increase in protein synthesis

5 liver enlargement

6 increase in phospholipid synthesis

7 increase in cytochromes P-450 content (3*)

8 increase in NADPH cytochrome P-450 reductase (3*)

9 increase in glucuronosyl transferases

10 increase in glutathione transferases

11 increase in epoxide hydrolases

12 induction of cytochrome P-450 mostly occurs in the centrilobular area of the liver.

The polycyclic hydrocarbon type of inducer does not have such major effects, only causing slight liver enlargement and having no effect on liver blood or bile flow. The increase in cytochromes P-450 is not confined to the centrilobular area of the liver, protein synthesis is only slightly increased and there is no increase in phospholipid synthesis. Other enzymes than cytochromes P-450 are also induced by polycyclic hydrocarbons, although generally to a lesser extent than with barbiturate induction. NADPH cytochrome P-450 reductase is not induced by polycyclic hydrocarbons however. With the clofibrate type of inducer other changes are also apparent. Thus, there is a proliferation in the number of peroxisomes (an intracellular organelle), as well as induction of a particular form of cytochrome P-450 involved in fatty acid metabolism. A number of other enzymes associated with the role of this organelle in fatty acid metabolism are also increased, such as carnitine acyltransferase and catalase. This phenomenon is discussed in more detail in Chapter 6. The onset of the inductive response is in the order of a few hours (3-6 h after polycyclic hydrocarbons, 8-12 h after barbiturates), is maximal after 3-5 days with barbiturates (24-48 h with polycyclic hydrocarbons) and lasts for at least 5 days (somewhat longer with polycyclic hydrocarbon induction). The magnitude of the inductive effect may depend on the size and duration of dosing with the inducer, and will also be influenced by the sex, species, strain of animal and the tissue exposed.

It is clear from these comments that the biochemical and toxicological effects seen after various inducers may be markedly different. This is illustrated by the effects of different inducers on the metabolism of various substrates examined in vitro and shown in table 5.20. It can be seen that in some cases the inducers cause no change in the metabolism, whereas in other cases metabolism is increased or even decreased. These effects are compounded by species and tissue differences in response to inducers and the differences in isoenzymes present in these species and tissues. Induction therefore may cause:

a increased rate of metabolism of a foreign compound through one pathway b altered metabolite profiles if the foreign compound is metabolized by several routes and only one is induced c no effect on metabolism if the particular isoenzymes induced are not involved in metabolism of the particular compound d decreased metabolism if induction increases levels of certain isoenzymes at the expense of the one (s) metabolizing the compound in question.

Depending on the role of metabolism in the toxicity of a compound therefore, enzyme induction may increase, decrease or cause no change in the toxicity of a particular

TABLE 5.20 Effect of various enzyme inducers on metabolism of different compounds


Compound_Control_Pb_PCN3MCAro nmolproduct/min/nmol cyt P-450 Ethylmorphine 13.7 16.8 24.9 6.4 9.5

Aminopyrine 9.9 13.9 9.7 7.613.7

Benzphetamine 12.5 45.7 6.6 5.715.8

Pb: phenobarbital; PCN: pregnenolone-16a-carbonitrile; 3MC: 3-methylcholanthrene; Aro: arochlor 1254. Data from Powis et al. (1977) In Microsomes and Drug Oxidations, edited by V.Ullrich, A.Roots, A.Hildebrandt, R.W. Estabrook and A.H.Conney, p. 137 (Oxford: Pergamon Press).

compound. The effects of induction need to be considered in the light of distribution and excretion as competing processes (figures 4.67 and 6.1). However, the consequences can be simply summarized and explained as follows:

i If a metabolite is responsible for the toxic effect of a compound, then induction of the enzyme responsible may increase that toxicity. However, if there is only one route of metabolism and elimination is dependent on this then only the rate of metabolism to the single metabolite will be increased rather than the total amount. This may not increase toxicity if no other factors are involved or are not time-dependent. However, as most toxic effects are multi-stage, involving repair and protection, this is unlikely.

ii If the parent compound is responsible for a toxic effect, then induction of metabolism may decrease that toxicity. However, induction of metabolism may lead to a different toxic effect due to a metabolite.

iii If a foreign compound is metabolized by several routes, then induction may alter the balance of these routes. This may lead to either increases or decreases in toxicity.

iv Induction may change the stereochemistry of a reaction.

Although some of these principles will be illustrated in more detail by the examples in Chapter 7, it is worthwhile examining some briefly at this point. A simple example is the pharmacological effect of a barbiturate, measured as sleeping time, which is dramatically reduced by induction of the enzymes of metabolism (table 5.21). This effect correlates with the plasma half-life, whereas the plasma level of pentobarbital on awakening is similar in the control and the induced groups (table 5.21). A toxicological example is afforded by paracetamol which is metabolized by several routes. The hepatotoxicity of paracetamol in the rat is increased by induction with phenobarbital due to an increase in the cytochrome P-450 isoenzyme which activates the drug. However, in the hamster hepatotoxicity is decreased due to an increase in glucuronidation, a detoxication pathway induced by phenobarbital (see Chapter 7). With the hepatotoxin bromobenzene, 3-methylcholanthrene induction decreases the

TABLE 5.21 Effect ofpentobarbital pretreatment on the duration of the pharmacological effect and disposition of pentobarbital in rabbits

PretreatmentSleeping time (min)

Plasma level of pentobarbital on

Pentobarbital half-life in plasma

awakening (^g/ml)


None 67±4



Pentobarbital 30±7



Rabbits were pretreated with three daily doses of pentobarbital (60 mg/kg; s.c.) then given a single challenge dose of pentobarbital (30 mg/kg; i.v.).

Data from Remmer (1962) In Metabolic Factors Controlling Duration of Drug Action, Proceedings of First International Pharmacological Meeting, Vol. 6, edited by B.B.Brodie and E.G.Erdos (New York: Macmillan).

Rabbits were pretreated with three daily doses of pentobarbital (60 mg/kg; s.c.) then given a single challenge dose of pentobarbital (30 mg/kg; i.v.).

Data from Remmer (1962) In Metabolic Factors Controlling Duration of Drug Action, Proceedings of First International Pharmacological Meeting, Vol. 6, edited by B.B.Brodie and E.G.Erdos (New York: Macmillan).

TABLE 5.22 Influence of cytochrome P-450 induction on the in vitro metabolism of R- and S-warfarin

Hydroxylated warfarin metabolites

^-isomer ^-isomer


7-OH 8-OH 7-OH (nmol warfarin metabolite/nmol P-450)



0.22 0.04 0.04 0.36 0.07 0.09 0.08 0.50 0.04

G.G1 G.G2 G.G4

Data from Gibson, G.G. and Skett, P. (1986) Introduction to Drug: Metabolism (London: Chapman & Hall).

Data from Gibson, G.G. and Skett, P. (1986) Introduction to Drug: Metabolism (London: Chapman & Hall).

toxicity. This is due to an increase in an alternative, non-toxic pathway, 2,3-epoxidation and an increase in the detoxication pathway catalysed by epoxide hydrolase (Chapter 7, table 7.12). With the lung toxic compound ipomeanol, phenobarbital induction decreases the toxicity, metabolic activation and the LD50, whereas pretreatment with 3-methylcholanthrene changes the target organ from the lung to the liver (see Chapter 7).

The pharmacological action of codeine is increased by induction as this increases demethylation to morphine. Induction by phenobarbital decreases the toxicity of organophosphates, but increases that of phosphorothionates. Studies with the drug warfarin have shown that induction by both phenobarbital and 3-methylcholanthrene will change the stereochemistry of the product, as can be seen in table 5.22. Thus, hydroxylation in the 8-position in the R-isomer is increased 12 times compared with only four times with the S-isomer following 3-methylcholanthrene induction.

Thus, the importance of enzyme induction is that it may alter the toxicity of a foreign compound. This can have important clinical consequences and underlie drug interactions. Thus, the antitubercular drug rifampicin is thought to increase the hepatotoxicity of the drug isoniazid, and alcohol may increase susceptibility to the hepatotoxicity of

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paracetamol. However, it should also be noted that induction can alter the metabolism of endogenous compounds. For example, the antitubercular drug, rifampicin, is a microsomal enzyme inducer in human subjects. As well as increasing the toxicity of drugs such as isoniazid this compound also alters steroid metabolism, and may lead to reduced efficacy of the contraceptive pill. Similarly, in birds exposed to chlorinated hydrocarbons induction of the enzymes involved in steroid metabolism is believed to lead to alterations in the production of eggs resulting in greater fragility and therefore increased breakage.

That the influence of environmental agents is sufficient to cause significant changes in xenobiotic metabolism in humans has been shown in a number of studies. This is illustrated by studies of the effects of cigarette smoking and cooking meat over charcoal on the metabolism of the drug phenacetin in human volunteers. Both these activities produce polycyclic hydrocarbons such as benzo/a/pyrene which is a potent microsomal enzyme inducer. Eating charcoal grilled steak was shown to cause a significant increase in the rate of metabolism of phenacetin by de-ethylation to paracetamol (figure 5.20). This was indicated by the plasma level of phenacetin, which was significantly lower (20-25%) in human volunteers after eating meat exposed to charcoal compared with foil-wrapped meat. There was no decrease in half-life as phenacetin undergoes a significant first-pass effect, and enzyme induction in the gastrointestinal tract may have been a factor in this study responsible for a significant proportion of the increased metabolism. Cigarette smoking similarly increased the rate of metabolism of phenacetin. A study comparing antipyrine metabolism in Caucasians with that in Asians revealed that there were significant differences in the rate of metabolism between the two ethnic groups. The greater rate of metabolism (shorter half-life, increased clearance) in the Caucasians was ascribed at least in part to the influence of dietary factors such as eating meat and exposure to coffee, cigarette smoke and alcohol in the Caucasians.

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  • Lisa
    How does enzyme induction affect acetaminophen metabolism?
    3 years ago
    What are enzyme induction and enzyme inhibition in toxicology?
    2 years ago
  • florian
    What effect does enzyme induction have on drugs?
    10 months ago
  • Petronio
    How does smoking cause microsomsl enze induction?
    6 months ago
  • emma genovese
    When does phenobarbital enzyme induction occur?
    5 months ago
  • tom goold
    How does cyclosporine affect metabolism by enzyme induction or inhibition?
    2 months ago

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