Xenobiotics, which are absorbed into biological systems by passive diffusion across membranes are usually lipid soluble and not ideally suited for excretion. Indeed very lipophilic substances may remain in the mammalian body for many years. After a xenobiotic has been absorbed, it may undergo biotransformation to products which are rapidly excreted and therefore elimination of the compound from the animal is facilitated. However, biotransformation may also change the biological activity of the substance. Hence, the metabolic fate of the compound can have an important bearing on its toxic potential, the disposition of the compound in the body and the excretion of the compound. The metabolic reactions involved are usually divided into phase 1 and phase 2 reactions, the latter being conjugation reactions. The products of metabolism are usually more water soluble than the original compound. Although usually detoxifying, these reactions, especially phase 1 ones, sometimes increase toxicity.


Keywords:

biotransformation;
metabolism;
Phase 1;
Phase 2;
conjugation;
cytochrome;
hydroxylation;
oxidation;
dealkylation

Biotransformation of Xenobiotics

Of the three-quarters of a million service personnel involved in the Persian Gulf War, approximately 30,000 have complained of neurological symptoms of unknown etiology. One contributing factor to the emergence of such symptoms may be the simultaneous exposure to multiple agents used to protect the health of service personnel, in particular, the anti-nerve agent pyridostigmine bromide (PB; 3-dimethylaminocarbonyloxy-N-methylpyridinium bromide), the insect repellent DEET (N,N-diethyl-m-toluamide), and the insecticide permethrin (3-(2,2-dichloro-ethenyl)-2,2-dimethylcyclopropanecarboxylic acid (3-phenoxyphenyl)methyl ester). This study investigated neurotoxicity produced in hens by individual or simultaneous exposure to these agents (5 d/wk for 2 months to 5 mg/kg/d PB in water, po; 500 mg/kg/d DEET, neat, sc; and 500 mg/kg/d permethrin in corn oil, sc). At these dosages, exposure to single compounds resulted in minimal toxicity. Combinations of two agents produced greater neurotoxicity than that caused by individual agents. Neurotoxicity was further enhanced following concurrent administration of all three agents. We hypothesize that competition for liver and plasma esterases by these compounds leads to their decreased breakdown and increased transport of the parent compound to nervous tissues. Thus, carbamylation of peripheral esterases by PB reduces the hydrolysis of DEET and permethrin and increases their availability to the nervous system. In effect, PB "pumps" more DEET and permethrin into the central nervous system. Consistent with this hypothesis, hens exposed to the combination of the three agents exhibited neuropathological lesions with several characteristics similar to those previously reported in studies of near-lethal doses of DEET and permethrin. If this hypothesis is correct, then blood and liver esterases play an important "buffering" role in protecting against neurotoxicity in the population at large. It also suggests that individuals with low plasma esterase activity may be predisposed to neurologic deficits produced by exposure to certain chemical mixtures.

Neurotoxicity resulting from coexposure to pyridostigmine bromide, deet, and permethrin: implications of Gulf War chemical exposures.

The past 60 years have seen a revolution in our understanding of the molecular genetics of insecticide resistance. While at first the field was split by arguments about the relative importance of mono- vs. polygenic resistance and field- vs. laboratory-based selection, the application of molecular cloning to insecticide targets and to the metabolic enzymes that degrade insecticides before they reach those targets has brought out an exponential growth in our understanding of the mutations involved. Molecular analysis has confirmed the relative importance of single major genes in target-site resistance and has also revealed some interesting surprises about the multi-gene families, such as cytochrome P450s, involved in metabolic resistance. Identification of the mutations involved in resistance has also led to parallel advances in our understanding of the enzymes and receptors involved, often with implications for the role of these receptors in humans. This Review seeks to provide an historical perspective on the impact of molecular biology on our understanding of resistance and to begin to look forward to the likely impact of rapid advances in both sequencing and genome-wide association analysis.

https://www.genetics.org/content/genetics/194/4/807.full.pdf

The Molecular Genetics of Insecticide Resistance

Chapter 16 – Carboxylesterases and Amidases

Arthropod pest resurgence: an overview of potential mechanisms

Two major classes of enzymes, i.e., hydrolases and transferases, comprise all the nonoxidative enzymes, and together these enzymes catalyze a wide variety of biotransformations of insecticides. The hydrolytic enzymes involved in insecticide metabolism are carboxylesterase (EC 3.1.1.1), arylesterase (EC 3,1.1.2), alkylamidase, and DFPase (EC 3.8.2.1). Recent experimental evidence suggests that carboxylesterase enzymes(s), formerly known to hydrolyze malathion-type insecticides, can also catalyze hydrolysis of a variety of diversified insecticidal esters such as benzilic acid derivatives, carbanilate compounds, and pyrethroids. These organo-phosphate-sensitive esterases, with the exception of the enzyme which hydrolyzes malathion, are all present in microsomes. Similarly, the action of amidases now extends to those insecticidal compounds or their intermediates which contain an aminoformyl (N--CHO) moiety. Arylesterase and DFPase catalyze the P--anhydride bond cleavage of the leaving group, a major hydrolytic pathway for organophosphate insecticides. Transferal enzymes which are presently known to metabolize insecticidal organophosphates are GSH-S-alkyltransferase (EC 2.5.1.12) and GSH-S-aryltransferase (EC 2.5.1.13). These enzymes cleave P--O--R (R = alkyl) or P--O--X (X = aromatic), with subsequent transfer of the R or X group to glutathione. Regarding the other conjugating enzymes, UDP-glucuronyltransferase (EC 2.4.1.17), UDP-glucosyltransferase (EC 2.4.1.35), and arylamine acetyltransferase (EC 2.3.1.5), much work is needed to understand their interactions with insecticidal compounds. There is some evidence that arylsulfotransferase (EC 2.8.2.1) may play a prominent role in the conjugative mechanisms of insects.

Nonoxidative enzymes in the metabolism of insecticides

Larval and adult housefly carboxylesterase: Isozymic composition and tissue pattern

Nadph oxidation by microsomal preparations of gypsy moth larval tissues

Fifth-instar gypsy moth, Lymantria dispar (L.), larvae reared on an artificial wheat germ diet show significantly ( P <0.001) higher mixed-function oxidase (MFO) activity than oak-reared larvae, as measured by gut microsomal NADPH oxidation, reduction of cytochrome c by NADPH-cytochrome- c -reductase, and N -demethylation of N,N -dimethyl- p -nitrophenyl carbamate. Also, diet-reared larvae, unlike oak-reared ones, exhibit a pre-ecdysial depression in MFO activity that parallels the pre-ecdysial cessation of feeding. The elevated MFO activity presumably results from certain inducers present in the artificial diet; these are either absent or occur at low levels in oak leaves. These findings have important implications with respect to comparative insect biochemistry and toxicology; e.g., assessment of MFO levels in different species, MFO induction studies, and insecticide tolerance and metabolism.

Gypsy Moth Mixed-Function Oxidases: Gut Enzyme Levels Increased by Rearing on a Wheat Germ Diet

Treatment of gypsy moth, Lymantria dispar (L.), larvae with the parasite Apanteles melanoscelus Ratzeburg and the microbial insecticide Bacillus thuringiensis Berliner yielded a higher mortality than treatment with A. melanoscelus or B. thuringiensis alone. The mortality was, however, less than the sum of the mortalities for the separate treatments. The 7- and 14-day LC50 values for parasitized (97.6 and 50.2 IU of B. thuringiensis per ml of diet, respectively) and for unparasitized (95.3 and 43.4 IU of B. thuringiensis per ml of diet, respectively) larvae showed no significant differences with respect to B. thuringiensis treatment. Mortality due to B. thuringiensis treatment continued up to the 28th day and was dependent on dosage. The majority of deaths due to parasitism alone occurred between the 15th and 28th days. B. thuringiensis treatment of host larvae caused an increase of ca. 3 days in the total development time of A. melanoscelus .

Sami Ahmad

Papers

Nonoxidative enzymes in the metabolism of insecticides

Larval and adult housefly carboxylesterase: Isozymic composition and tissue pattern

Nadph oxidation by microsomal preparations of gypsy moth larval tissues

Gypsy Moth Mixed-Function Oxidases: Gut Enzyme Levels Increased by Rearing on a Wheat Germ Diet

Toxicity of Bacillus thuringiensis to Gypsy Moth Larvae Parasitized by Apanteles melanoscelus