Antimetabolites competitive enzyme inhibitors

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. 

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.). 

A. Metabolic activation The antimetabolites (and their subsequent enzymic reaction product(s), respectively) may be utilized as competitive substrate(s) in one (or several consecutive) enzymic reaction(s) along the metabolic pathway of the normal metabolite, but at one stage of the metabolic reaction sequence, the transformed analogue cannot be further utilized as a substrate and, instead, acts as an inhibitor of the enzyme which catalyzes the next reaction step. At this stage, the action of the transformed ( activated ) analogue as an enzyme inhibitor depends on the same general types of structural requirements as outlined in the case of the directly acting enzyme inhibitors (see Section 2.2. ... 

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). 

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. 

The mode of action of sulfanilamides became known around 1947, when the structure and biosynthesis of folic acid were elucidated. This compound is built by bacteria from the heterocyclic pteroyl moiety, p-aminobenzoate, and glutamate. p-Aminobenzene-sulfonamide (9.89, sulfanilamide) is a competitive inhibitor of the synthase enzyme, acting as an antimetabolite of p-aminobenzoate. Occasionally, the sulfanilamide can even be incorporated into the modified folate, resulting in an inactive compound and thus an inactive enzyme. This theory, proposed by Woods and Fildes in 1940, became the first molecular explanation of drug action. 

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,... 

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,... 

Sulfonamides The sulfonamides are bacteriostatic inhibitors of folic acid synthesis. As antimetabolites of PABA, they are competitive inhibitors of dihydropteroate synthase (Figure 46-1). They can also act as substrates for this enzyme, resulting in the synthesis of nonfunctional forms of foUc acid. The selective toxicity of sulfonamides results from the inability of mammalian cells to synthesize folic acid they must use preformed folic acid that is present in the diet. 

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. 

---