Don L. Bull

Metabolic detoxification: mechanism of insect resistance to plant psoralens.

Larvae of the black swallowtail butterfly, Papilio polyxenes Stoll, forage successfully on plants that contain high levels of photosensitizing psoralens. These insects rapidly detoxify psoralens, particularly in the midgut tissue prior to absorption, with the result that appreciable levels of unmetabolized phototoxin do not enter the body circulation where deleterious light-induced interactions with dermal or subdermal tissues would occur.

Isolation and characterization of a diuretic peptide common to the house fly and stable fly

An identical CRF-related diuretic peptide (Musca-DP) was isolated and characterized from whole-body extracts of the house fly, Musca domestica, and stable fly, Stomoxys calcitrans. The peptide stimulates cyclic AMP production in Manduca sexta Malpighian tubules and increases the rate of fluid secretion by isolated Musca domestica tubules. The 44-residue peptide, with a mol.wt. of 5180, is amidated, and has the primary structure: NKPSLSIVNPLDVLRQRLLLEIARRQMKENTRQVELNRAILKNV-NH2. Musca-DP has a high percentage of sequence identity with other characterized CRF-related insect diuretic peptides.

In vitro metabolism of a linear furanocoumarin (8-methoxypsoralen, xanthotoxin) by mixed-function oxidases of larvae of black swallowtail butterfly and fall armyworm

Studies were made of the comparative in vitro metabolism of [14C]xanthotoxin and [14C]aldrin by homogenate preparations of midguts and bodies (carcass minus digestive tract and head) of last-stage larvae of the black swallowtail butterfly (Papilio polyxenes Fabr.) and the fall armyworm [Spodoptera frugiperda (J. E. Smith)]. The two substrates were metabolized by 10,000g supernatant microsomal preparations from both species. Evidence gained through the use of a specific inhibitor and cofactor indicated that mixed-function microsomal oxidases were major factors in the metabolism and that the specific activity of this enzyme system was considerably higher in midgut preparations fromP. polyxenes than in similar preparations fromS. frugiperda. Aldrin was metabolized 3–4 times faster byP. polyxenes, and xanthotoxin 6–6.5 times faster.

FATE OF PHOTOSENSITIZING FURANOCOUMARINS IN TOLERANT AND SENSITIVE INSECTS

The fate of [14C]xanthotoxin (8-methoxypsoralen) was studied in larvae of insect species that are tolerant (Papilio polyxenes Stoll) or sensitive (Spodoptera frugiperda J.E. Smith) to the phototoxic effects of photosensitizing psoralens. Both insects metabolize xanthotoxin by oxidative cleavage of the furan ring, but the detoxification occurs at a much more rapid rate inP. polyxenes in which >95% of an oral 5 μg/g xanthotoxin dose is metabolized within 1.5 hr after treatment. The detoxification of psoralens byP. polyxenes appears to occur primarily in the midgut tissue prior to absorption, with the result that the intact phototoxin does not reach appreciable levels in body tissues. Studies with an angular furanocoumarin indicated that isopsoralens are metabolized byP. polyxenes at a somewhat slower rate than observed for psoralens; however, a reduced rate of metabolic detoxification of isopsoralens probably does not explain the fact that psoralen tolerance inP. polyxenes does not extend to the isopsoralen series.

Comparative toxicity, absorption, and metabolism of chlorpyrifos and its dimethyl homologue in methyl parathion-resistant and -susceptible tobacco budworms☆☆☆★

Abstract Chlorpyrifos (Dowco 179) and its dimethyl homologue, chlorpyrifosmethyl (Dowco 214), were used to study the influence of the O,O-dialkyl group of organophosphorus insecticides on toxicity, absorption, and metabolism among larvae of the tobacco budworm [Heliothis virescens (F.)] from strains that were resistant (R) and susceptible (S) to methyl parathion. In toxicity tests, chlorpyrifos and chlorpyrifosmethyl were more toxic than methyl parathion to 3rd-stage R larvae but less toxic to S larvae. Chlorpyrifosmethyl was more toxic (3–4 ×) than chlorpyrifos to both strains of larvae, and the results of absorption studies indicated that the toxicity differential of the homologues may be explained in part by the more rapid absorption of the dimethyl form. Studies of the in vivo metabolism of both Dowco compounds indicated that each was degraded mainly by the cleavage of the pyridylphosphate linkage. In vitro tests demonstrated that the NADPH-dependent microsomal oxidases were of primary importance in detoxification, while glutathione (GSH)-dependent mechanisms (aryl- and alkyltransferases) present in the soluble cell fractions were of lesser importance. O-dealkylation occurred only with chlorpyrifosmethyl. The R larvae demonstrated greater capability in detoxifying both compounds in the comparative in vivo and in vitro studies of metabolism, but the differences were more apparent during the 5th instar than during the 3rd instar.

In vivo and in vitro fate of fenvalerate in house flies

Investigations of pyrethroid resistant (R) and susceptible (S) house flies (Musca domestica L.) provided conclusive evidence that a high level of cross-resistance to fenvalerate in the laboratory-selected R strain was influenced by an enhanced capability for metabolic degradation of the toxicant. Comparative studies of the in vitro metabolism of [14C]fenvalerate by homogenate preparations of S and R house flies, in the presence or absence of the mixed function oxidase (mfo) inhibitor piperonyl butoxide (PBO) or the esterase inhibitor S,S,S-tributylphosphorothioate (DEF), indicated that elevation of mfo activity was a major component of enhanced metabolism and that there also was enhancement of esterases associated with both the microsomal and cytosolic subcellular fractions of homogenate preparations of R insects. In vitro studies of the effects of coad-ministered PBO or DEF on the absorption and metabolism of topically applied [14C]fenvalerate in R insects demonstrated that both synergists slowed the rate at which the toxicant penetrated the cuticle; PBO significantly reduced the metabolic degradation of fenvalerate, but effects of DEF on metabolism were less conclusive.

Comparative metabolism of [3H]psoralen and [3H]isopsoralen by black swallowtail (Papilio polyxenes Fabr.) caterpillars

The comparative fate of tritiated preparations of a linear furanocoumarin (psoralen) and an angular furanocoumarin (isopsoralen) was determined in last-instar caterpillars of the black swallowtail butterfly (Papilio polyxenes Fabr.). Oral administration of either furanocoumarin at 5 μg/g is followed by rapid metabolism, primarily through oxidative cleavage of the furan ring, and the metabolites are rapidly excreted. Isopsoralen is, however, metabolized at a somewhat slower rate than is psoralen, and levels of unmetabolized isopsoralen in body tissues of the treated caterpillars are about three-fold higher. These data are compatible with the hypothesis that a reduced detoxification rate accounts at least in part for the susceptibility ofP. polyxenes caterpillars to the deleterious effect of isopsoralens.

Activity and fate of diflubenzuron and certain derivatives in the boll weevil

Abstract Adult female boll weevils ( Anthonomus grandis Boheman) were treated topically or fed treated diets containing either (i) diflubenzuron ( N -[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide) or two of its hydroxylated derivatives or (ii) methylated analogs of these three chemicals. Only diflubenzuron and N -methyl diflubenzuron inhibited the hatch of eggs laid by the treated insects. Comparative studies of the fate of 14 C-labeled diflubenzuron (I), N -methyl diflubenzuron (IV), 2-methoxy- N -methyl diflubenzuron (VII), and 3-methoxy- N -methyl diflubenzuron (VIII) after topical treatment (1 μg each) of female boll weevils demonstrated the following: (i) the four chemicals were absorbed, metabolized, and excreted at different rates in the order IV > VII, VIII > I; (ii) the chemicals were secreted into eggs and initial concentrations of 2.24 (I), 2.82 (IV), 1.47 (VII), and 0.93 ppm (VIII) at 4 days post-treatment declined progressively in subsequent samples; and (iii) concurrent daily observations indicated that inhibition of egg hatch by I and IV seemed to diminish rapidly when their concentrations in eggs declined to ca. 1.0 ppm or lower.

Target site and enzyme changes associated with selection of subcolonies of a multiresistant house fly strain with methyl parathion or permethrin

Abstract The effects of intensified selection pressure on subcolonies of a multiresistant house fly strain were determined with studies of different parameters associated with the responses of insects to insecticides. This study included three resistant strains (R, routinely selected by dual exposure to malathion and permethrin; Rmp, a substrain of R selected intensively with methyl parathion; Rper, a substrain of R selected intensively with permethrin); and S, an insecticide-susceptible strain. The Rmp and Rper strains were tested after selection of 20 consecutive generations. Topical toxicity tests indicated resistance of the Rmp strain to methyl parathion increased 17-fold compared with the parental R strain, but there was only a slight increase in tolerance to permethrin. Resistance to permethrin increased 2.6-fold in the Rper flies while tolerance to methyl parathion was unchanged. Compared with the S insects, there was a substantially decreased sensitivity of brain AChE to in vitro inhibition by methyl paraoxon in all three resistant strains. The order of AChE sensitivity to inhibition among strains was S > Rper = R > Rmp. Susceptible or resistant isomeric forms of AChE, respectively, were homogeneous in the S and Rmp strains, but were a heterogeneous mixture in R and Rper insects. Studies of the specific binding of [ 3 H]saxitoxin by brain membrane receptors demonstrated significant differences between susceptible and resistant strains in sodium channel density; the order of maximal binding capacity ( B max ) was S > R = Rmp = Rper. In vitro studies of key enzyme systems demonstrated that, compared with the parental R strain, there were substantial increases in titers of microsomal oxidases and glutathione- S -transferases in the Rmp and Rper flies. There were no major differences among the three resistant strains in cuticular penetration of trans -permethrin or in titers of DDT-dehydrochlorinase or permethrin esterase. All these enzyme systems were more active in the three resistant strains than in S insects.