Enzyme inhibition/inhibitors antimetabolite

Donald Woods discovered that sulphonamides exerted their action by inhibiting an enzyme used by bacteria to synthesise folic acid. The compound 4-aminobenzoic acid is the precursor for folic acid, and is structurally similar to sulphonamide. Bacteria that were unable to synthesise folic acid were unable to achieve de novo synthesis of purines for their DNA and RNA synthesis and hence could not proliferate. Such competitive inhibitors, which mimicked normal metabolites, became known as antimetabolites (many are used in cancer chemotherapy. Chapter 21). 

Antimetabolites are enzyme inhibitors (see p. 96) that selectively block metabolic pathways. The majority of clinically important cytostatic drugs act on nucleotide biosynthesis. Many of these are modified nucleobases or nucleotides that competitively inhibit their target enzymes (see p. 96). Many are also incorporated into the DNA, thereby preventing replication. 

Antimetabolite inhibitors of DNA synthesis act by the competitive or allosteric inhibition of a number of different enzymes participating in purine or pyrimidine biosynthesis. Actually, some such compounds interfere with as many as 10-12 different enzymes— although admittedly to a different degree. 

Mammals must obtain their tetrahydrofolate requirements from their diet, but microorganisms are able to synthesize this material. This offers scope for selective action and led to the use of sulphanilamide and other antibacterial sulpha drugs, compounds which competitively inhibit dihydropteroate synthase, the biosynthetic enzyme incorporating p-aminobenzoic acid into the structure. These sulpha drugs thus act as antimetabolites of p-aminobenzoate. Specific dihydrofolate reductase inhibitors have also become especially useful as antibacterials,... 

A. The antimetabolite plays the role of a substrate If the antimetabolite is capable of undergoing the enzyme-catalyzed reaction with the resulting dissociation of the enzyme-antimetabolite complex into (abnormal) produces) and the free enzyme, then it may be considered an abnormal substrate, or substitute metabolite. As such, it will competitively interfere with the transformation of the normal metabolite the extent of such interference depends on the relative affinity of the antimetabolite for the enzyme as well as on the rate of its conversion and subsequent release by the enzyme (i.e., the turn-over rate of the enzyme-antimetabolite complex). In the extreme (but important) case when the affinity is very high and the turnover rate very low, such antimetabolites act, in effect, as potent enzyme inhibitors, rather than as substitute metabolites (see B.iii) below). In the majority of cases, those classical antimetabolites which are capable of undergoing the enzyme-catalyzed reaction, having affinities and conversion rates comparable to those of the corresponding normal metabolites, exert only a partial and temporary inhibition at those steps of the metabolic pathway in which they themselves are metabolized, and therefore, their effective action as metabolic inhibitors will depend on their inhibition of other targets and on subsequent metabolic events (see Section 2.3.). 

Methotrexate, a common antimetabolite, was introduced several decades ago for the treatment of psoriasis and remains an effective therapeutic approach. It is a synthetic analogue of folic acid that acts as a competitive inhibitor of the enzyme dihydrofolate reductase, that is responsible for the conversion of dihydrofolate to tetrahydrofolate. Tetrahydrofolate is an essential cofactor for the synthesis of thymidy-late and purine nucleotides required for DNA and RNA synthesis. Methotrexate inhibits replication and function of T and B cells and suppresses secretion of various cytokines such as IL-1, IFN-y,... 

Control of parasitic invasions has been successfully achieved with compounds that inhibit key enzymes in pathways vital to the biosynthesis of the building blocks of nucleic acids in these pathogenic microorganisms. This results in their reproductive suppression or death. An alternative mechanism can also exist, particularly if the inhibitor has a close chemical similarity to the enzyme s normal substrate (metabolite). Such an analog compound may still be affected by the enzyme, resulting in a false structural component, which if incorporated into the essential biopolymer at all, will lead to nonfunctioning (or errant) DNA or RNAs. Such drugs are referred to as antimetabolites. They... 

All susceptible fungi are capable of deaminating flucytosine to 5-fluorouracil, a potent antimetabolite that is used in cancer chemotherapy. Fluorouracil is metabolized fast to 5-fluorouracil-ribose monophosphate (5-FUMP) by the enzyme uracil phosphodbosyi transferase (UPRTase, also called uridine monophosphate pyrophosphorylase). As in mammalian cells, 5-FUMP then is either incorporated into RNA (via synthesis of 5-fluorouridine triphosphate) or metabolized to 5-fluoro-2 -5 deoxyuridine-5-monophos-phate (5-FdUMP), a potent inhibitor of thymidylate synthetase. DNA synthesis is impaired as the ultimate inhibition of this latter reaction. The selective action of flucytosine is due to the lack or low levels of cytosine deaminase in mammalian cells, which prevents metabolism to fluorouracil. 

Pentostatin is a purine antimetabolite. It is a potent transition-state inhibitor of the enzyme adenosine deaminase (ADA) that leads to cytotoxicity because of elevated intracellular levels of dATP that can block DNA synthesis through inhibition of ribonucleotide reductase. Pentostatin can also inhibit RNA synthesis as well as cause increased DNA damage. It is indicated in the treatment for both untreated and alpha-interferon-refractory hairy-cell leukemia and as palliative therapy of chronic lymphocytic leukemia, prolympho-cytic leukemia and cutaneous T-cell lymphoma. 

Dihydropteroate synthase Sporozoans (eg, plasmodium, toxoplasma, and eimeria species) lack the ability to utilize exogenous folate and therefore possess enzymes for its synthesis these enzymes can be inhibited by drugs. Sulfonamides, which are antimetabolites of PABA, inhibit dihydropteroate synthase. Sequential blockade ctin be achieved with a sulfonamide and an inhibitor of dihydrofolate reductase (eg, pyrimethamine) such drug combinations are effective in malaria and toxoplasmosis. 

Allopurinol, a xanthine oxidase inhibitor used for the treatment of gout, inhibits metabolism of 6-mercaptopurine and other drugs metabolized by this enzyme. A serious drug interaction results from the concurrent use of allopurinol for gout and 6-mercaptopurine to block the immune response from a tissue transplant or as antimetabolite in neoplastic diseases. In some cases, however, allopurinol is used in conjunction with 6-mercaptopurine to control the increase in uric acid elimination from 6-mercaptopurine metabolism. The patient should be supervised closely, because when given in large doses, allopurinol, an inhibitor of purine metabolism, may have serious effects on bone marrow. 

The antimetabolite, raltitrexed, is a folate analogue and is a potent and specific inhibitor of the enzyme thymidylate synthase. Inhibition of this enzyme ultimately interferes with the synthesis of deoxyribonucleic acid (DNA) leading to cell death. The intracellular polyglutamation of raltitrexed leads to the formation within cells of even more potent inhibitors of thymidylate synthase. Folate (methylene tetrahydrofolate) is a co-faetor required by thymidylate synthase and therefore theoretically folinic acid or folic acid may interfere with the aetion of raltitrexed. Clinieal interaction studies have not yet been undertaken to confirm these predieted inter-aetions. ... 

In a similar way the purine antimetabolite 6-mercapto-purine (6-MP)/ after conversion to the monophosphoribo-tide, acts as an inhibitor of this enzyme (9.10). There have been described other sites of action of 6-MP (4-7) which are marked by arrows in figure 1. We have previously demonstrated the inhibition of the enzymic formate activation (tetrahydrofolate formylase) in leukemic cells by 6-MP (12). inspite of a rather high inhibitory concentration of 6-MP between 10 and 10 M, this inhibition has some practical clinical implications for the treatment of acute leukemia/ as it is detected in sensitive leukemic cells only. In accordance with reports of several authors (8,9/10) we have postulated/ that 6-MP has to be converted into 6-thioinosinic acid for exerting its inhibitory effect on the de novo synthesis of purine-nucleotides. On the other hand DAVIDSON and WINTER have shown that both 6-MP sensitive and resistant cells con-... 

Tyrosine hydroxylase Inhibitors - Perhaps the greatest effort is being directed toward studies of the enzyme tyrosine hydroxylase which represents the rate limiting step in catecholamine synthesis. This enzyme catalyzes the conversion of tyrosine to DOPA and Is localized In the particulate fraction of the cell sedimenting at 16,000 x g. Inhibition of this enzyme has been found to be the most effective means of blocking the formation of norepinephrine. Much of the biochemistry of this and other enzymes associated with catecholamines was discussed at the Second Symposium on Catecholamines and published in 1966. Hundreds of compoumds have been examined for anti-tyrosine hydroxylase activity, but only a few exhibited Inhibitor activity vitro, and these were mainly analogues of tyrosine or its catechol metabolites. It has become apparent, however, that there is not always a relationship between vitro and to vivo activity. Of the many compounds thus far tested, two which have shown In vivo activity are a-methyl-1-tyroslne (a-MT) and H44/68 (the methyl ester-HCl of c MT), both acting as competitive antimetabolites of tyrosine. 

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