PRINCIPLES OF TOXICOLOGY - Biology East Borneo

78 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATIONof acetylcholine esterase by organophosphates is beneficial if it is being used as a pesticide, but not ifit is directed against humans.Most studies of inhibition of xenobiotic metabolism have centered on cytochrome P450. Earlystudies identified a compound, SKF 525A, as one of the first cytochrome P450 inhibitors, and althoughused extensively in laboratory investigations, it has no therapeutic use. Like most cytochrome P450inhibitors, much of its effect can be attributed to it being an alternative substrate, namely, a competitiveinhibitor. Some compounds exhibit noncompetitive characteristics. Many of these are heme ligands,which do not bind to the apoprotein “substrate site,” and this class includes many nitrogenousheterocyclic compounds such as substituted pyridines, N-substituted imidazoles, and triazoles. Carbonmonoxide, although an inhibitor of cytochrome P450 via heme binding, does not do so in vivo becauseit is sequestered in the blood before reaching the liver. In addition to the two classes described above,a third group of inhibitors that exhibit both of the abovementioned characteristics has been described.Their dual nature arises from their being substrates for metabolism initially, but the products of thatmetabolism either disrupt the protein structure (e.g., chloramphenicol, cyclophosphamide) or hemefunction. Heme function can be compromised by alkylation of the heme (e.g., dihydropyridines,unsaturated compounds such as olefins), which produces green pigments, by covalent linking of theheme to the protein (carbon tetrachloride), or by binding as ligands to the heme iron. This lattersubgroup includes methylenedioxybenzene derivatives such as isosafrole and piperonyl butoxide andmany amines including SKF 525A, troleandomycin, and related compounds. The complexes they formare classified as cytochrome P450 metabolic-intermediate complexes, and these can be detected bytheir characteristic ferrous state absorbance spectrum around the same wavelength as seen with thecarbon monoxide, about 450 nm.Because both competitive and suicide (mechanism-based) inhibitors require active-site recognition,inhibitors can be extremely selective for the enzyme or isozyme they inhibit. Some such selectivecytochrome P450 isozyme inhibitors are given in Table 3.4. For some compounds, the exact nature oftheir inhibition of cytochrome P450 remains obscure; ethanol is one such example. Despite all theinformation available on drug interactions and toxic episodes resulting from inhibition, it is likely thatthe mechanism(s) of many of them have yet to be fully elucidated.The biological consequences of inhibition of metabolism are two fold. In the acute phase,interactions can manifest themselves as either the potentiation of the biological effect of each, ifmetabolism results in inactivation, or protection from toxicity if toxicity arises from the bioactivationof the parent molecule. With chronic exposure, many agents generally considered as inhibitors (e.g.,SKF 525A and clotrimazole) are also inducing agents (see Table 3.6). It appears that the compensationfor long-term cytochrome P450 inhibition can be induction, perhaps as a response designed tocircumvent the block. It should be noted, however, that more xenobiotic metabolizing enzymes thancytochrome P450 are induced by cytochrome P450 inhibitors. The induction seen with chronicexposure to inhibitors can thus result in drug interactions that are opposite to those listed as acuteeffects.In addition to substrate-binding (active) site inhibition, drug metabolizing capability can be reducedby cosubstrate or cofactor depletion (e.g., glutathione, SO 4, NAD + ), by their diversion to otherbiochemical pathways, or by an inhibition of enzymes responsible for their formation. In laboratoryinvestigations, glutathione conjugation can be inhibited by either buthionine sulfoximine, whichinhibits the synthesis of glutathione; or diethylmaleate, which sequesters available glutathione.Galactosamine, prior to its hepatotoxic effect can deplete UDPGA by sequestering UTP. For multicomponentreactions, the xenobiotic metabolism reaction can be inhibited at a distance (e.g., cytochromeP450 oxidations can be inhibited by the interruption of electron flow by heavy-metal ions,such as mercury, because the flavoprotein contains a more susceptible sulfhydryl group). Sincexenobiotic metabolism is catalyzed by enzymes, many of the reactions can be inhibited nonselectivelyby protein denaturants such as heavy-metal ions and detergents, the degree of inhibition depending onthe concentration. For enzymes that require a suitable membrane environment for activity, xenobioticswith lipid solvent properties can inhibit activity by destroying that necessary environment. Changesin lipid often lead to conformational changes that alter activity.