More recently it has become apparent that genetic factors also affect phase 1, oxidation pathways. Early reports of the defective metabolism of diphenylhydantoin in three families and of the defective de-ethylation of phenacetin in certain members of one family indicated a possible genetic component in microsomal enzyme-mediated reactions. Both these cases resulted in enhanced toxicity. Thus, diphenylhydantoin, a commonly used anticonvulsant, normally undergoes aromatic hydroxylation and the corresponding phenolic metabolite is excreted as a glucuronide (figure 7.35). Deficient hydroxylation
results in prolonged high blood levels of diphenylhydantoin and the development of toxic effects, such as nystagmus, ataxia and dysarthria. The deficiency in the ability to hydroxylate diphenylhydantoin is inherited with dominant transmission.
The defective de-ethylation of phenacetin was discovered in a patient suffering methaemoglobinaemia after a reasonably small dose of the drug. This toxic effect was observed in a sister of the patient but not in other members of the family. The metabolism of phenacetin in the patient and in the sister was found to involve the production of large amounts of the normally minor metabolites 2hydroxyphenacetin and 2-hydroxyphenetidine, with a concomitant reduction in the excretion of paracetamol, the major metabolic product of de-ethylation in normal individuals (figure 5.20). It was suggested that autosomal recessive inheritance was involved, with the 2-hydroxylated metabolites probably responsible for the methaemoglobinaemia.
These early observations suggesting that genetic factors affect the oxidation of foreign compounds were confirmed by studies on the metabolism of the anti-hypertensive drug debrisoquine. The benzylic oxidation in the 4-position of the alicyclic ring (figure 5.21) has been found to be defective in 5-10% of the white population of Europe and North America. This is detected as a bimodal distribution when the metabolic ratio, urinary 4-hydroxydebrosoquine: debrisoquine, is plotted against frequency of occurrence in the population (figure 5.22). The two phenotypes detectable are known as poor (PM) and extensive (EM) metabolizers and the poor metabolizer phenotype behaves as an autosomal recessive trait. Thus the extensive metabolizers are either homozygous (DD) or heterozygous (Dd) and the poor metabolizers are homozygous (dd). Poor metabolizers suffer an exaggerated pharmacological effect after a therapeutic dose of the drug as a result of a higher plasma level of the unchanged drug (figure
5.23). Extensive metabolizers excrete 10-200 times more 4-hydroxydebrisoquine than poor metabolizers. The deficiency extends to more than 20 different drugs, and various types of
metabolic oxidation reaction. For example, the aromatic hydroxylation of guanoxon, S-oxidation of penicillamine, oxidation of sparteine (figure 5.24) and the hydroxylation of bufuralol (figure 5.1) have all been shown to be polymorphic. There are now a number of adverse drug reactions which are associated with the poor metabolizer status. For example perhexiline may cause hepatic damage and peripheral neuropathy in poor metabolizers in lÛÛy lÛÛy
whom the half-life is significantly extended; penicillamine causes skin rashes, haematuria and thrombocytopenia in poor metabolizers; lactic acidosis may be associated with the use of phenformin in poor metabolizers.
The biochemical basis for the trait is an almost complete absence of one form of cytochrome P-450, CYP 2D6. It seems that there are several mutations which give rise to the poor metabolizer phenotype. These
FIGURE 5.24 Compounds known to show the same oxidation polymorphism in humans as debrisoquine. The arrows indicate the site of oxidation.
mutations produce incorrectly spliced, variant mRNAs in the liver from poor metabolizers, and two mutant alleles have been identified which are linked to the poor metabolizer phenotype. However, for some of the substrates of the polymorphism, such as bufuralol, biphasic kinetics has been demonstrated in vitro. This data and data from studies using inhibitors such as quinidine, which is specific for the debrisoquine hydroxylase form of cytochromes P-450, has indicated that two functionally different forms of cytochrome P-450 isoenzymes, are involved. These two forms differ in that one has low affinity whereas the other has high affinity. Only one of these may be under polymorphic control. The metabolism of bufuralol is further complicated by the fact that it is stereospecific. The aliphatic hydroxylation (1-hydroxylation) is selective for the (+) isomer, whereas the aromatic 4- and 6-hydroxylations are selective for the (-) isomer. Both aliphatic and aromatic hydroxylation of bufuralol are under the same genetic control, and the selectivity is virtually abolished in the poor metabolizer. Only the high affinity, polymorphic enzyme is stereospecific; the low affinity enzyme is not. Studies with sparteine have also indicated that there are two isoenzymes involved in metabolism to 2-and 5-dehydrosparteine. Thus, there is a high affinity, quinidine sensitive form and a low affinity, quinidine insensitive form. The formation of the metabolites by the two isoenzymes is, however, quantitatively and qualitatively different. Both isoenzymes exhibit large interindividual differences, but it is believed that the low affinity enzyme is not controlled by the debrisoquine polymorphism. The variation in the high affinity isoenzyme is suggested as being due to allozymes of cytochrome P-450; that is, different enzymes from the alleles comprising one gene.
The importance of such polymorphisms in human susceptibility to diseases, such as cancer, is now increasingly being recognized. For example, studies in humans with amino biphenyl, a liver and bladder carcinogen, have shown that both the acetylator phenotype and hydroxylator status are important in the formation of adducts (see Chapter 6).
Another similar, but distinct, genetic polymorphism concerns the aromatic 4-hydroxylation of the drug
mephenytoin (figure 5.25). This hydroxylation deficiency
FIGURE 5.25 Aromatic hydroxylation of mephenytoin.
occurs in 2-5% of Caucasians and in 20% of Japanese. Again it is an autosomal recessive trait. The enzyme, cytochrome P-450, form CYP 2C, from poor metabolizers has a high Km and low Fmax. As with the hydroxylation of bufuralol, the hydroxylation is stereoselective. Thus, only S-mephenytoin undergoes aromatic 4-hydroxylation and only this route is affected by the polymorphism.
Was this article helpful?
This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.