PRINCIPLES OF TOXICOLOGY

78 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATION
of acetylcholine esterase by organophosphates is beneficial if it is being used as a pesticide, but not if
it is directed against humans.
Most studies of inhibition of xenobiotic metabolism have centered on cytochrome P450. Early
studies identified a compound, SKF 525A, as one of the first cytochrome P450 inhibitors, and although
used extensively in laboratory investigations, it has no therapeutic use. Like most cytochrome P450
inhibitors, much of its effect can be attributed to it being an alternative substrate, namely, a competitive
inhibitor. 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 nitrogenous
heterocyclic compounds such as substituted pyridines, N-substituted imidazoles, and triazoles. Carbon
monoxide, although an inhibitor of cytochrome P450 via heme binding, does not do so in vivo because
it 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 that
metabolism either disrupt the protein structure (e.g., chloramphenicol, cyclophosphamide) or heme
function. 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 the
heme to the protein (carbon tetrachloride), or by binding as ligands to the heme iron. This latter
subgroup includes methylenedioxybenzene derivatives such as isosafrole and piperonyl butoxide and
many amines including SKF 525A, troleandomycin, and related compounds. The complexes they form
are classified as cytochrome P450 metabolic-intermediate complexes, and these can be detected by
their characteristic ferrous state absorbance spectrum around the same wavelength as seen with the
carbon 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 selective
cytochrome P450 isozyme inhibitors are given in Table 3.4. For some compounds, the exact nature of
their inhibition of cytochrome P450 remains obscure; ethanol is one such example. Despite all the
information available on drug interactions and toxic episodes resulting from inhibition, it is likely that
the 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, if
metabolism results in inactivation, or protection from toxicity if toxicity arises from the bioactivation
of 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 compensation
for long-term cytochrome P450 inhibition can be induction, perhaps as a response designed to
circumvent the block. It should be noted, however, that more xenobiotic metabolizing enzymes than
cytochrome P450 are induced by cytochrome P450 inhibitors. The induction seen with chronic
exposure to inhibitors can thus result in drug interactions that are opposite to those listed as acute
effects.
In addition to substrate-binding (active) site inhibition, drug metabolizing capability can be reduced
by cosubstrate or cofactor depletion (e.g., glutathione, SO 4 , NAD + ), by their diversion to other
biochemical pathways, or by an inhibition of enzymes responsible for their formation. In laboratory
investigations, glutathione conjugation can be inhibited by either buthionine sulfoximine, which
inhibits the synthesis of glutathione; or diethylmaleate, which sequesters available glutathione.
Galactosamine, prior to its hepatotoxic effect can deplete UDPGA by sequestering UTP. For multicomponent
reactions, the xenobiotic metabolism reaction can be inhibited at a distance (e.g., cytochrome
P450 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). Since
xenobiotic metabolism is catalyzed by enzymes, many of the reactions can be inhibited nonselectively
by protein denaturants such as heavy-metal ions and detergents, the degree of inhibition depending on
the concentration. For enzymes that require a suitable membrane environment for activity, xenobiotics
with lipid solvent properties can inhibit activity by destroying that necessary environment. Changes
in lipid often lead to conformational changes that alter activity.