C.F. Wilkinson

Detoxication Enzymes in the Guts of Caterpillars: An Evolutionary Answer to Plant Defenses?

Higher activity of midgut microsomal oxidase enzymes in polyphagous than in monophagous species indicates that the natural function of these enzymes is to detoxify natural insecticides present in the larval food plants. Differing strategies of adaptation to plant defenses may partly account for the great diversity of insect herbivores.

Herbivore-Plant Interactions: Mixed-Function Oxidases and Secondary Plant Substances

The mixed-function oxidases of a polyphagous insect larva (the southern armyworm, Spodoptera eridania) were found to be induced by a diversity of secondary plant substances. The induction proceeds rapidly and in response to a small quantity of secondary substance. Following induction, the larva is less susceptible to dietary poisoning. It is argued that mixed-function oxidases play a major role in protecting herbivores against chemical stress from secondary plant substances.

Microsomal mixed-function oxidases in insects—I: Localization and properties of an enzyme system effecting aldrin epoxidatton in larvae of the southern armyworm (Prodenia eridania)☆

Abstract An extremely active microsomal epoxidasc has been obtained from larvae of the southern armyworm ( Prodenia eridania ) and optimal assay conditions in vitro have been established. Maximum enzyme activity is associated primarily with the larval mid-gut. Specific activity of the enzyme changes markedly during larval development and reaches a maximum in the sixth instar.

Substituted imidazoles as inhibitors of microsomal oxidation and insecticide synergists

Abstract The inhibitory activity of forty-seven 1-, 2-, and 4(5)-substituted imidazoles toward microsomal epoxidation, hydroxylation, and N -demethylation in enzyme preparations from rat liver and armyworm gut has been evaluated. Many of the 1- and 4(5)-substituted compounds are among the most potent inhibitors of microsomal expoxidase and hydroxylase yet reported but are uniformly less active toward N -demethylation. The armyworm gut enzymes are generally more susceptible to inhibition than those from rat liver. Inhibition follows either competitive or mixed type kinetics depending on both the inhibitor and substrate involved. The imidazoles bind powerfully to cytochrome P-450 and yield type II difference spectra in oxidized and NADPH-reduced microsomes. Since spectral dissociation constants ( K s ) appear to closely parallel I 50 values, the inhibitory activity of the imidazoles is considered to result mainly from their capacity to bind to both cytochrome P-450 and closely related substrate binding sites. Several of the 1-arylimidazoles are potent in vivo synergists of carbaryl against NAIDM insecticide susceptible houseflies.

Structure-activity relationships in the effects of 1-alkylimidazoles on microsomal oxidation in vitro and in vivo

The inhibitory activity of a series of 13 1-alkylimidazoles toward microsomal epoxidation of aldrin in enzyme preparations from rat liver and armyworm (Prodenia eridania) gut is optimal in compounds with a chain length of 8–10 carbon atoms. The capacity of the imidazoles to bind to cytochrome P-450 (type II) appears to be closely related to their inhibitory activity. The activity of the compounds in vivo in synergizing the toxicity of carbaryl to houseflies and potentiating pentobarbital sleeping time in mice closely parallels the data in vitro. Regression analyses clearly establish that both activity patterns in vitro and in vivo can be satisfactorily described by linear equations in terms of the hydrophobic bonding constant (π and π2) indicating a close correlation between biological activity and lipophilic character.

Steric factors in the inhibitory interaction of imidazoles with microsomal enzymes

Abstract The ability of substituted imidazoles to bind to cytochrome P-450 (type II) and to inhibit microsomal drug-metabolizing enzyme activity is highly dependent on the position of the substituent in the imidazole ring. Compounds containing substituents in the 1-, 4(5)- and 1,5-positions are effective inhibitors of the epoxidation of aldrin in rat liver microsomes and armyworm ( Spodoptera eridania ) gut preparations and exhibit low spectral dissociation constants ( K s . In contrast, compounds containing 2- and/or 4-substitutents are essentially inactive as inhibitors and exhibit high K s values (low binding). It appears that both binding and inhibition depend on the accessibility of the non-bonded electrons on the nitrogen atom at position — 3 of the ring.

Induction of microsomal enzymes in the southern armyworm (Prodenia eridania)

Abstract Levels of microsomal epoxidation, N-demethylation, and cytochrome P-450 in the gut tissues of sixth instar southern armyworm larvae were considerably enhanced following oral in vivo treatment with a series of methylbenzenes. Induction increased with increasing methyl substitution and was maximal with pentamethylbenzene. The increase in microsomal activity occurred rapidly after initiation of treatment and the final levels of induction achieved were dependent on the concentration of the inducer in the diet and the time of exposure. Microsomal enzyme activity returned to control levels following termination of exposure and induction was blocked by puromycin and cycloheximide but not by actinomycin D. The in vivo tolerance of induced worms to orally administered carbaryl was increased in a manner reflecting the enhanced microsomal enzyme activity.

Ah receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin: Comparative studies in mammalian and nonmammalian species

Abstract The Ah receptor was identified and characterized in cytosol from a large number of diverse animal species. Molecular properties of the Ah receptor were similar in all species which had detectable receptor. However, subtle differences in receptor properties exist among animal species and these differences indicate that the Ah receptor protein is not identical in all species. It is not yet known whether differences in receptor properties among animal species play a significant role in determining differential susceptibility of the species to toxicity from halogenated aromatic compounds.

Effects of dihydrosafrole on cytochromes P-450 and drug oxidation in hepatic microsomes from control and induced rats

Changes in cytochromes P-450, aminopyrine N-demethylase (APDM), aromatic hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), and type III spectral complex formation were measured in hepatic microsomes of control, phenobarbital (PB)-, and beta-naphthoflavone (beta NF)-induced rats after a single dose of dihydrosafrole (4-n-propyl-1,2-methylenedioxybenzene, DHS). Time profiles of changes in these microsomal parameters were complex and showed that APDM activities and cytochrome P-450 levels decreased immediately after treatment and were associated with concurrent increases in the intensity of the type III methylenedioxyphenyl (MDP) metabolite/cytochrome P-450 spectral complex. In noninduced rats, both APDM activity and cytochrome P-450 levels returned to control levels between 12 and 24 hr after treatment with DHS and subsequently increased above control levels. In PB- and beta NF-induced animals, the inhibitory phases were more prolonged and activity never returned to levels higher than the corresponding controls. AHH activity was increased substantially (two- to three-fold) in all cases after DHS administration. Although displacement of the MDP metabolite/cytochrome P-450 complex with 2-methylbenzimidazole generally led to a marked restoration of cytochrome P-450 levels and partially reversed the inhibition of APDM, it had little or no effect on AHH activities.

Juvenile Hormone Analogs: A Possible Case of Mistaken Identity?

Many synthetic compounds that exhibit activity in juvenile hormone assays have limited structural resemblance to natural juvenile hormones. The observed morphogenic action of many of the compounds considered to be biologically analogous to juvenile hormones is probably synergistic rather than intrinsically hormonal.