False substrate

Dihydropteroic acid (85) is an intermediate to the formation of the folic acid necessary for intermediary metabolism in both bacteria and man. In bacteria this intermediate is produced by enzymatic condensation of the pteridine, 86, with para-amino-benzoic acid (87). It has been shown convincingly that sulfanilamide and its various derivatives act as a false substrate in place of the enzymatic reaction that is, the sulfonamide blocks the reaction by occupying the site intended for the benzoic acid. The lack of folic acid then results in the death of the microorganism. Mammals, on the other hand, cannot synthesize folic acid instead, this compound must be ingested preformed in the form of a vitamin. Inhibition of the reaction to form folic acid Ls thus without effect on these higher organisms. 

Despite the large size of an enzyme molecule, there is reason to believe that there are only one or a few spots on its surface at which reaction can occur. These are usually referred to as active centers. The evidence for this view of enzyme reactions comes from many kinds of observations. One of these is that we can often stop or slow down enzyme reactions by adding only a small amount of a false substrate. A false substrate is a molecule that is so similar to the real substrate that it can attach itself to the active center, but sufficiently different that no reaction and consequently no release occurs. Thus, the active center is blocked by the false substrate. 

A PRACTICAL APPLICATION OF ENZYME INHIBITION BY A FALSE SUBSTRATE... 

Usually fairly high concentrations of such a drug are needed for effective control of an infection because the inhibitor (the false substrate) should occupy as many active centers as possible, and also because the natural substrate will probably have a greater affinity for the enzyme. Thus the equilibrium must be influenced and, by using a high concentration of the false substrate, the false substrate-enzyme complex can be made to predominate. The bacteria, deprived of a normal metabolic process, cannot grow and multiply. Now the body s defense mechanisms can take over and destroy them. 

Purines and Related Heterocycles Considerable research has been devoted to preparation of modified purines in the expectation that such compounds could act as antagonists to, or possibly false substrates for, those involved in normal metabolic processes. It is surprising to note the relatively small number of such compounds that have found clinical use. 

The rate-limiting step in the synthesis of the catecholamines from tyrosine is tyrosine hydroxylase, so that any drug or substance which can reduce the activity of this enzyme, for example by reducing the concentration of the tetrahydropteridine cofactor, will reduce the rate of synthesis of the catecholamines. Under normal conditions tyrosine hydroxylase is maximally active, which implies that the rate of synthesis of the catecholamines is not in any way dependent on the dietary precursor tyrosine. Catecholamine synthesis may be reduced by end product inhibition. This is a process whereby catecholamine present in the synaptic cleft, for example as a result of excessive nerve stimulation, will reduce the affinity of the pteridine cofactor for tyrosine hydroxylase and thereby reduce synthesis of the transmitter. The experimental drug alpha-methyl-para-tyrosine inhibits the rate-limiting step by acting as a false substrate for the enzyme, the net result being a reduction in the catecholamine concentrations in both the central and peripheral nervous systems. 

ACE inhibitors, such as captopril and enalaprilat, the active metabolite of enalapril, occupy the enzyme as false substrates. Affinity significantly influences efficacy and rate of elimination. 

Inhibition of nucleobase synthesis (2). Tetrahydrofolic acid (THF) is required for the synthesis of both purine bases and thymidine. Formation of THF from folic acid involves dihydrofolate reductase (p. 272). The folate analogues aminopterin and methotrexate (ame-thopterin) inhibit enzyme activity as false substrates. As cellular stores of THF are depleted, synthesis of DNA and RNA building blocks ceases. The effect of these antimetabolites can be reversed Ltillmann, Color Atlas of Pharmacology 2000 Thieme All rights reserved. Usage subject to terms and conditions of iicense. 

False substrates, 161 Famotine, 37 Fantridone, 421 Fenalamide, 81 Fenbufen, 126 Fenclorac, 66 Fenclozic acid, 269 Fenestrel, 9 Fenimide, 237... 

Methyldopa is an false substrate for the dopamine-/ -hydroxylase resulting in a-methylnor-adrenaline. This metabolite is an a2-adrenoceptor agonist an induce, like clonidine, a centrally mediated reduction of sympathetic tonus. 

Exemestane is known uniquely as an aromatase inactivator. It acts as a false substrate for the aromatase enzyme, and is processed to an intermediate that binds irreversibly to the active site of the enzyme causing its irreversable inactivation. Exemestane is used for the treatment of hormonally-responsive breast cancer in postmenopausal women. It is generally well tolerated and adverse events are usually mild to moderate. Adverse events include hot flushes, nausea, fatigue and increased appetite. 

Acetylcholinesterase can be inhibited by two general mechanisms. In the first mechanism, positively charged quaternary ammonium compounds bind to the anionic site and prevent ACh from binding—a simple competitive inhibition. In the second mechanism, the agents act either as a false substrate for the cholinesterase or directly attack the esteratic site in both cases they covalently modify the esteratic site and non-competitively prevent further hydrolytic activity. Either mechanism can be effective in preventing the hydroly-... 

Glycosylated Compounds. The rationale for the use of modified nucleosides for the treatment of cancer and viral disease relies on the hope that cancer cells or virally infected cells will mistake the modified compounds for the natural substrates and incorporate them into a metabolic pathway. The altered stmc-ture of the false substrate will, it is hoped, then bring that process to a halt and result in cell death. 

The insertion of a second oxygen atom in a sugar furanose ring in essence converts that moiety to an acetal. This modiftcation leads to another false substrate for viral reverse transcriptase. Glycosilation of the silylated purine (46-1) with chiral dioxolane (46-2) prepared in several steps from anhydromannose in the presence of ammonium nitrate affords the coupling product as a mixture of anomers (46-3). The mixture of products is then separated on a chromatographic column. The desired diastereomer (46-3) is reacted with ammonia to afford the product (46-4)... 

TIs also inhibit the reverse transcriptase enzyme s ability to perform one of the initial steps in HIV replication. The NNRTIs, however, directly inhibit the active (catalytic) site on this enzyme, whereas zidovudine and other NRTIs serve as false substrates that take the place of the substance (thymidine) normally acted on by this enzyme (see Reverse Transcriptase Inhibitors Mechanism of Action ). Hence, NNRTIs provide another way to impair one of the key steps in HIV replication, and these drugs can be used along with other agents (NRTIs, protease inhibitors) to provide optimal benefits in preventing HIV replication and proliferation (see the next section). 

The possibility of phosphorus analogues of amino-acids acting as false substrates and so interfering with biological mechanisms led us to consider the a-amino-phosphonous acids 4 as perhaps the closest analogues. 

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