Parkinson's disease and cytochrome P450: A possible link?

03M-9817/90/0032-@277/$lO.W

Medical Hyporhcses (1990) 32.277-212 0 Lawman Grotm UK Ltd 1990

Parkinson’s Disease and Cytochrome Possible Link? G. S. SHAHI,

N. P. DAS*

P450: A

and S. M. MOOCHHALA’

Departments of Physiology, *Biochemistry and ‘Pharmacology, National University of Singapore, Kent Ridge Crescent, Singapore 0517 /Correspondence and reprint requests to GSSI

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Abstract - The possibility that idiopathic Parkinson’s disease may be linked to a deficiency of an important detoxifying enzyme such as the cytochrome P450 enzyme bufuralol hydroxylase is examined. A hypothetical model of how this might operate to produce early onset Parkinson’s disease is suggested.

Introduction The recent discovery that the dopaminergic neurotoxin MPTP (I-methyl-4-phenyl-1,2,3,6tetrahydropyridine) can cause a syndrome clinically indistinguishable from idiopathic Parkinson’s disease (1) has aroused considerable interest in the possibility that Parkinson’s disease may be due to an MPTP-like environmental neurotoxin (2). Variations in individual susceptibility to the effects of MPTP suggest differences in detoxifying enzyme activity (3). Epidemiologic studies on the incidence of Parkinson’s disease point to a correlation between poor metabolizers of debrisoquine and the early onset of Parkinson’s disease (4). These ‘poor metabolizers’ are homozygous for an autosomal recessive gene resulting in deficiency of a form of the

Date received 11 July 1989 Date accepted 20 November 1989

cytochrome P450 enzyme, bufuralol hydroxylase (P450 buf l), and, hence, have a reduced capacity to hydroxylate drugs such as debrisoquine and bufuralol (5). These facts suggest that deficiency of appropriate detoxifying enzymes may predispose to Parkinson’s disease. This paper assesses the evidence that idiopathic Parkinson’s disease may be associated with such a deficiency in the cytochrome P450 system and hypothesizes how this might result in Parkinson’s disease. The cytochrome P450 system The cytochrome P450 enzyme system consists essentially of a havoprotein, NADPH cytochrome P450 reductase, and a family of related but distinct hemoproteins collectively called cytochrome P450 (6). NADPH cytochrome P450 reductase (EC 1.6.2.4) IS . responsible for the transfer of electrons from NADPH to cytochrome P450 (7) which then serves as the terminal monooxygenase for the metabolism of a wide vari-

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MEDICAL HYPOTHESES

ety of xenobiotics and endogenous substrates such as steroids and fatty acids (8). For full activity of the enzyme system after reconstitution from the isolated proteins, the lipid phosphatidylcholine is also required (9). While the exact function of the lipid is not known, it may provide the microenvironment necessary for efficient interaction of the protein components (6) and appears to facilitate electron transfer from NADPH-cytochrome P450 reductase to cytochrome P450 (10). Cytochrome P450 activity has traditionally been studied principally in the liver since it is found here in the highest concentrations (11). However, the cytochrome P450 system has also been found to be active in renal cortical tissue (12, 13), intestinal mucosa (14), lung (15), skin (16) and in practically all tissues tested (17) including the brain (18).

Fig. 1

Metabolic

pathways

of MPTP

Cytochrome P450 in the brain centration in hog liver (28) and has also been found in mouse (29) and rat (30) 3 liver as well as in rabbit lung (31). It appears likely that this enzyme may play a significant role in detoxication of MPTP in liver but it does not appear to be present in measurable quantities in mammalian brain (32) and thus is unlikely to be of significance in brain metabolism and detoxication of MPTP (33). Studies using the cytochrome P450 inhibitors SKF 525A and metyrapone have shown that such inhibition results in increased MPTP-induced hepatocyte cytotoxicity (34) - thus suggesting a possible protective role for cytochrome P450 against MPTP toxicity. Other studies suggest that a specific isozyme of cytochrome P450, an enzyme with bufuralol hydroxylase activity (the human form of which is called P450 buf l), is responsible for the metabolism of MPTP in the liver and brain (24) and interacts selectively with MPTP in the brain (35).

The brain as a whole contains approximately 1% of the amount of cytochrome P450 of that found in the liver (19) with great variations in regional distribution - with the highest content in the cerebellum (approximately 4x the average in the brain) and the lowest in the substantial nigra (approximately 20% of the brain average) (19). It appears likely that at least some of the cytochrome P450 in the brain is involved in the metabolism of endogenous substrates such as the aromatization of androgens (20) and the 2-hydroxylation of estrogen (21). Brain P450 also participates in the oxidation of several xenobiotics such as aminopyrine (22) 7 ethoxycoumarin (23) and bufuralol (24).

MPTP and cytochrome P450 The parkinsonism-causing neurotoxin MPTP can be metabolised by several pathways - the most important ones are shown in figure 1. The metabolism of MPTP by MAO B and the resultant formation of the toxic metabolites MPDP+ and MPP+ have been well characterised (25, 26). In addition, MPTP is also metabolised by the microsomal enzyme flavin monooxygenase to give MPTP N-oxide (27) and by cytochrome P450 to give 4-phenyl-1,2,3,6_tetrahydropyridine, PTP (28). Both these pathways are thought to result in the detoxication of MPTP (27). Flavin monooxygenase is found in high con-

Bufuralol hydroxylase and Parkinson’s disease Studies by Barbeau et al. (4) have shown a correlation between individuals who are .poor metabolizers of debrisoquine and the early onset of Parkinson’s disease. Poor metabolizers are homozygous for an autosomal recessive gene resulting in deficiency of cytochrome P450 buf 1 (24) and thus have a reduced capacity to metabolise drugs such as debrisoquine and bufuralol

278

I’AKliINSON‘S

DISEASE

AND CYTOCHROME

P450: A POSSIBLE

LINK’.

The question that remains to be answered is this: What possible link is there between the cytochrome P450 enzyme system and the possibility that an MPTP-like neurotoxin might be the cause of Parkinson’s disease? MPP.

Hypothesis

p.l.qU.1

Fig. 2

Similarity in structure between MPP+, the toxic metabolite of MPTP. and the insecticide paraquat

MPTP, bufuralol hydroxylase and early-onset Parkinson’s disease

This enzyme has been shown to be present in rat and human brain (24). How could deficiency of cytochrome buf 1 lead to the early onset of Parkinson’s disease? (5).

Parkinson’s disease and MPTP has long been established that idiopathic Parkinson’s disease is characterised pathologically by death of neurons in the zona compacta of the substantia nigra. However, the mechanism by which this occurs has still not been elucidated (36). Epidemiological work suggests that Parkinson’s disease is not primarily a genetic disorder (37, 38) but point, instead, to a possible environmental etiology (39, 40). The discovery that the neurotoxin MPTP causes selective damage to the dopaminergic neurons of the zona compacta of the substantia nigra (41) and produces a syndrome in humans very similar to idiopathic Parkinson’s disease (1) has given further impetus to the possibility that idiopathic Parkinson’s disease might well be environmental in origin and caused by an MPTP-like neurotoxin found in the environment (2). Already, several new compounds having effects similar to MPTP have been found (42) and a search is being hotly pursued for possible environmental parkinsonism-causing neurotoxins (3). In this context, it is interesting to note the similarity in structure between MPP+, the toxic metabolite of MPTP,’ the paraquat, an insecticide (Fig. 2), and the finding by Rajput et al. (40) of an increased incidence of early Parkinson’s disease in agricultural areas where insecticides such as paraquat are used in large amounts. If Parkinsons disease is indeed due to an MPTP-like neurotoxin, an insight into the processes of bioactivation and detoxication of MPTP would be essential if a means of preventing the disease is to be found. It

It is known that individual susceptibility to the neurotoxic effects of MPTP varies greatly and has thus been proposed that this may be due to differences in detoxifying enzyme activity (3). Figure 3 illustrates our conception of the interaction between environmental exposure to MPTP or an MPTP-like neurotoxin and detoxication by bufuralol hydroxylase or another important detoxifying enzyme in normal and enzymedeficient individuals. Massive exposure to neurotoxin as occurs by experimental injection of MPTP is highly unlikely to occur in nature - chronic low level exposure is much more likely to be a factor of aetiological significance in Parkinson’s disease (43). Low level exposure to neurotoxin can be expected to result in its detoxication by the detoxifying cytochrome P450 enzyme. High dose exposure which overwhelms the detoxication mechanism (2), or exposure in an individual with deficiency of the necessary detoxifying enzyme would result in failure of detoxication and a much increased likelihood of bioactivation of MPTP and the resultant formation of toxic metabolites. Thus, as can be predicted by analysis of this schema, individuals deficient in the requisite P450 detoxifying enzymes would be at increased risk of Parkinson’s disease and are likely to manifest symptoms earlier than individuals with normal P450. It is important to note, however, that individuals with normal P450 are not exempt from Parkinson’s disease and are likely to manifest signs and symptoms of the disease if exposure to large amounts of neurotoxin occurs. This schema would help explain how poor metabolizers of debrisoquine (that is, P450 buf 1 deficient individuals) might develope Parkinson’s disease of early onset as was noted by Barbeau et al. (4). It would also help explain the finding of Rajput et al. (40) of the greater risk of early Parkinson’s disease in agricultural areas where, presumably, exposure to neurotoxin is greater.

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MEDICAL HYPOTHESES

Fig. 3 A)

B)

C)

Conclusion

Changes in dopamine level with age in a normal person who has never been exposed to MPTP or an MPTP-like neurotoxin. Note that brain dopamine levels below about 20% would result in the person displaying the signs and symptoms of Parkinson’s disease. As can be seen, this level of dopamine is usually not reached in the course of a normal lifespan Sudden decrease in brain dopamine levels in a person who has been acutely exposed to a large dose of MPTP or an MPTP-like neurotoxin at age 40 years resulting in brain dopamine levels below 20% and, hence, the manifestations of Parkinson’s disease. This person might, for instance, be a drug addict who has selfadministered synthetic heroin contaminated with a high concentration of MPTP The effects of chronic low-dose environmental exposure to an MPTP-like neurotoxin on brain dopamine levels in a person with normal levels of enzymes involved in the detoxication of the toxin (dotted line) and in a person with reduced ability to detoxify the toxin (eg due to deficiency of the cytochrome P-450 enzyme, bufuralol hydroxylase, as proposed in text). In the example shown, exposure occured between the ages of 5 and 35. Both the normal person and the person with a deficiency in the appropriate detoxifying enzymes experienced an increased likelihood of damage to dopaminergic neurons due to “toxic stress” and a resultant fall in brain dopamine levels. The person with the enzyme deficiency will, however, be at greater risk and is likely to have an accelerated destruction of dopaminergic neurons. It can be seen that both individuals might now develope Parkinson’s disease but the person with the enzyme deficiency is more likely to manifest Parkinson’s disease of The available epidemiologic early onset. evidence points in favour of such a mechanism (see text).

The possibility that environmental manipulation P450 sys- might help prevent the debility that goes with tem might play a major role in protection of an Parkinson’s disease is an exciting one and worthy individual from the effects of environmental of further attention. neurotoxins. Thus, while Parkinson’s disease may not be a primarily genetic disease and exAcknowledgements posure to as yet undiscovered environmental toxins may be necessary in its pathogenesis, The authors wish to express their gratitude to MS Shoon Mei genetic deficiency of important detoxifying en- Yin for preparing the illustrations used in this manuscript. zymes such as the cytochrome P450 enzyme bufuralol hydroxylase may predispose to References Parkinson’s disease. The cytochrome P450 sysI. Ballard P A, Tetrud J W, Langston J W. Permanent tem and its integrity would thus be of major human parkinsonism due to 1-methyl-4-phenyl-1.2,3.6significance in determining the likelihood that an tetrahydro-pyridine (MPTP): seven cases. Neurology individual is at increased risk of Parkinson’s dis35: 949-956.1985 ease due to environmental exposure to toxins. 2. Shahi G S, Moochhala S M, Lee E J D. Das N P. 280 It, thus, appears that the cytochrome

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P450: A POSSIBLE

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