Inhibition of Penicillium duponti carboxylesterase by the fungicides captan and folpet

Captan, folpet, and perchloromethylmercaptan were effective inhibitors of Penicillium duponti p-nitrophenylpropionate esterase activity (I₅₀ = 0.5 – 2 μM) whereas α-naphthyl acetate esterase activity was not affected by the presence of these compounds. Captan and folpet are both equally effective at pH 7.3 and 8.3. The ionic composition of the medium had strong effects on the degree of inhibition produced by all inhibitors but did not alter esterase activity. Neither succinamide nor phthalimide caused inhibition of the p-nitrophenylpropionate esterase activity: The trichloromethylmercaptan portion of these fungicides appears to be responsible for the observed inhibition. The rapidity of captan and folpet inhibition of esterase activity (complete in < 1 min) compared to the rates of spontaneous decomposition ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{> 1}\text{min}$) and the insensitivity of captan and folpet inhibition to hydrogen ion concentration suggest that generation of spontaneous decomposition products is not required for inhibition. The results are consistent with a mechanism in which the entire fungicide molecule binds to the protein followed by enzyme-promoted reactions of captan and folpet which result in loss of esterase activity.     

Electrochemical characterization and redox reaction scheme of the diphenyl ether herbicide nitrofen

Redox behavior of nitrofen (2,6-dichlorophenyl-p-nitrophenyl ether) at a glassy carbon working electrode in aprotic and protolytic solvents was examined using cyclic voltammetry. The electrode reaction of 2 mM nitrofen in dimethylformamide (0.1 M tetra-n-butylammonium hexafluorophosphate as supporting electrolyte) was quasireversible. Measured half-wave potential was ?1.11 V vs saturated calomel reference electrode (SCE) and peak potential separation was 0.110 V. The reduction product was chemically stable on the time scale of the experiment; cathodic peak current equalled anodic peak current. Linear sweep voltammetry employing a rotating, glassy carbon disk electrode indicated that electron stoichiometry for reduction of nitrofen in dimethylformamide and acetonitrile was one. Reduction of nitrofen under aprotic conditions was most likely a one-electron reduction to the nitro radical anion. Reduction of 1 mM nitrofen in 50% ($\text{v}\text{v}$) ethanol/water buffered at pH 5.0, 7.0, and 10.2 with 0.25 M sodium acetate, 0.1 M potassium phosphate, and 0.1 M sodium carbonate was chemically irreversible and characteristic of an EC … -type reaction mechanism; heterogeneous electron transfer followed by homogeneous chemical reactions. Cathodic peak potentials at pH 5.0 and 7.0 were ?0.760 and ?0.820 V vs SCE. At pH 10.2 two cathodic peaks were observed at ?0.840 and ?1.17 V vs SCE. In protolytic solvent at pH 5.0 cathodic peak current varied linearly as a function of the square root of the scan rate. Thus, the electrode reaction was limited by diffusion. Electron stoichiometry for reduction of nitrofen in protolytic solvent was four. The fate of new redox species generated in protolytic solvent at pH 7.0 was examined upon subsequent multiple scans at 200 mV/s. Initial reduction at ?0.820 V vs SCE was most likely centered at the nitro group, yielding the four-electron hydroxylamine product. Oxidation at 0.020 V vs SCE was thought to correspond to the two-electron oxidation of hydroxylamine to the nitroso derivative.     

The metabolism of p,p′-DDT by the feral pigeon (Columba livia)

Male feral pigeons were dosed with ring-labeled $\text{[}{}^{14}\text{C}\text{]p,p′-}\text{DDT}$ and the tissues and droppings analyzed for total ¹⁴C, extractable ¹⁴C, and metabolites. Only 16% of an intraperitoneal dose of 1.5–2.2 mg kg^?1 was voided in the droppings over 28 days; the rate of loss reached a maximum on the 14th day and then fell quickly away. The rate of removal of ¹⁴C in droppings was low in comparison to that found in the rat and the Japanese quail. When pigeons were dosed with 32–38 mg kg^?1 DDT per bird, and killed after 77 days, 5.4% of the dose was eliminated in droppings and 87% was recovered in the body. The tissues and droppings from this experiment were analyzed for DDT and its metabolites. Of the ¹⁴C remaining in tissues 88% was accounted for as the apolar compounds DDE, DDT, and DDD. Approximately half of the ¹⁴C in droppings was present as DDE, DDT, and DDD, whereas 27–35% was apparently in conjugated form, extractable from aqueous solutions by ethyl acetate after prolonged acid hydrolysis. Two polar metabolites were isolated from the acid-released material. One was p,p′-DDA; the other was extractable from aqueous solution at pH 8 and was tentatively identified as a monohydroxy derivative of p,p′-DDT. DDE accounted for 93% of the ¹⁴C present as metabolites in tissues and droppings, clearly indicating the importance of this intermediate in this study. The metabolism of DDT in the feral pigeon is discussed in relation to its metabolism by other species.     

Mechanism of inhibition reaction of acetylcholinesterase by phenyl N-methylcarbamates: Separation of hydrophobic,electronic, hydrogen bonding and proximity effects of aromatic substituents

The association equilibrium constant, $\text{1}\text{K}{}_{\mathrm{d}}$, and the carbamylation constant, k₂, of 53 o-, m-, and p-substituted phenyl N-methylcarbamates with bovine erythrocyte acetylcholinesterase were determined. The $\text{1}\text{K}{}_{\mathrm{d}}$ value varied 1000-fold, whereas the k₂ value did not depend upon the nature and position of substituents. The variation in $\text{log}\text{(}\text{1}\text{K}{}_{\mathrm{d}}\text{)}$ was analyzed using free energy related substituent parameters and regression analyses. The effect of substituents at o-, m-, and p-positions was nicely separated into hydrophobic, electronic, hydrogen bonding, and proximity (steric and field electronic for o-substituents) factors. The physicochemical significance of these factors was established by comparison with those for model organic reactivities. The mechanism of the whole reaction process was elucidated in terms of physical organic chemistry.     

Identification of products arising from the metabolism of cis- and trans-chlordane in rat liver microsomes in vitro: Outline of a possible metabolic pathway

The metabolism of pure cis- and trans-chlordane was studied in vitro. Microsomal preparations from the livers of male rats induced with cis- or trans-chlordane in feed for 10 days were used to metabolize the pure compound corresponding to the inducer. Subsequent extraction, column fractionation, and combined gas chromatography-mass spectroscopy resulted in the characterization of four compounds not previously reported from an in vitro system. In addition to the substrate, trans-chlordane extracts contained species with the following molecular weights and empirical formulas: $\text{m}\text{e}$ 370, C₁₀H₅Cl₇, heptachlor; $\text{m}\text{e}$ 352, C₁₀H₆OCl₆, a hydroxylated chlordene; and $\text{m}\text{e}$ 422, C₁₀H₆OCl₈, a hydroxylated chlordane. Dichlorochlordene, oxychlordane, and 1-chloro-2-hydroxy-dihydrochlordene were also present. With the exception of the hydroxychlordane, cis-chlordane extracts contained all of the metabolites found in the trans incubates. Additionally, a fully saturated compound, $\text{m}\text{e}$ 372, C₁₀H₇Cl₇, a dihydroheptachlor, was present. The 1,2-trans-dihydrodiol of heptachlor found in previous in vitro incubates of cis-chlordane was not present in this extract. This information has been incorporated into a proposed route for the biotransformation of the chlordanes that offers an explanation for the observed differences in the metabolism of cis and trans isomers. The pathway is based on the reductive dechlorination of the chlordanes through dihydroheptachlor to dihydrochlordene. Parallel pathways of hydroxylation, desaturation, and epoxide formation arise at each of these species and at chlordane itself.     

Metabolism of lindane in house flies: Metabolic desaturation,dehydrogenation and dehydrochlorination,and conjugation with glutathione

In lindane-treated house flies, a cis-dehydrogenated metabolite, $\text{(}\text{36}\text{45}\text{)-}\text{hexachlorocyclohexene}$, was identified by gas-liquid chromatography and mass spectrometry. The in vitro metabolism study showed that in the presence of NADPH the microsomal fraction of house flies converted lindane to three hexane-soluble metabolites. This conversion was inhibited by piperonyl butoxide, SKF-525A, and carbon monoxide. These metabolites were identified as $\text{(}\text{36}\text{45}\text{)-}\text{hexachlorocyclohexene}$, $\text{(}\text{36}\text{45}\text{)- and (}\text{346}\text{5}\text{)-pentachlorocyclohexene}$ (PCCHE) by gas-liquid chromatography. They, as well as lindane, were excellent substrates for the reaction with the postmicrosomal fraction in the presence of glutathione. While the reaction with lindane-d₆ showed a significant deuterium isotope effect (6.82), that of $\text{(}\text{36}\text{45}\text{)-}\text{PCCHE-d}{}_5$ did not (1.18). Enzymatic conjugation with glutathione probably occurs at the stage of PCCHE.     

Mitochondrial changes during storage of untreated or CIPC-treated potatoes

During storage of potato tubers (Solanum tuberosum L., var. Bintje), important changes appear which affect respiratory control, $\text{ADP}\text{O}$, intensity of O₂ consumption in the presence of different substrates, and NAD⁺ dependence, In mitochondria extracted under strictly similar conditions, from patato tubers stored at 4°C, the respiratory control (RC) maintains a value near 4 for 4 to 5 months. It then declines progressively to low values. At 20°C, a stable RC of 4 can be observed for several months, which then decreases at the end of dormancy. Then, the RC increases sharply; at this stage, $\text{ADP}\text{O}$ are abnormally low, and, some time later, NAD⁺ dependence disappears. Mitochondria treated with 250 μM chlorpropham show a 50% inhibition of the electron transfer with exogenous NADH as substrate. After tuber treatment with 1% chlorpropham, sprouting is inhibited for several months. The activities of mitochondria extracted from such tubers remain unaffected by the treatment. The use of this phenylcarbamate for potato tuber treatment permits obtaining functional mitochondria from tubers after a slightly longer period of storage.     

Components of the electron transport system in the microsomal mixed-function oxidase system in wild birds

Notable differences were found among six species of wild-caught birds in the levels of cytochrome P-450, cytochrome b₅, NADPH-cytochrome c reductase, and NADH-cytochrome c reductase. Ethyl isocyanide difference spectra showed significant variations among the species in peak height and in the ratios of the $\text{430}\text{455}\text{-}\text{nm}$ peaks. Substantial aldrin epoxidase activity was found in all species, and the amounts of dieldrin produced compared favorably with pigeon and rat liver microsomes. Higher content of cytochrome P-450 was not always accompanied by a similar rise in specific catalytic activity. Thus, no correlation could be established between these two parameters. Aldrin epoxidase activity with NADH as the sole electron donor was 25–49% as effective as with the NADPH-generating system. Addition of both NADH and NADPH-generating systems to the incubation mixture produced a synergistic effect with liver microsomes of two species but not with two other species. DDE and polychlorinated biphenyls residues were found in the heart tissue of all species examined, and this might indicate a possible inductive effect on the microsomal mixed-function oxidase system by environmental contaminants.     

Spectral and inhibitory interactions of methylenedioxyphenyl compounds with southern armyworm (Spodoptera eridania) midgut microsomes

Characteristics of the Type III optical difference spectra of 13 methylenedioxyphenyl compounds in NADPH-fortified armyworm midgut microsomes varied with the nature of the substituents in the aromatic ring. Compounds with electron-donating substituents yielded spectra with large $\text{427}\text{458}\text{nm}$ peak ratios, whereas those with electron-withdrawing groups exhibited low $\text{427}\text{458}\text{nm}$ peak ratios. Small amounts of carbon monoxide were generated during incubation of the 4,5-dihalo derivatives with midgut microsomes, and cis- and trans-methylenedioxycyclohexanes exhibited spectra with a major Soret peak at about 430 nm and a very weak absorbance maximum at about 480 nm. Formation of the Type III spectral complex occurred very rapidly and was associated with a marked decrease (up to 72%) in cytochrome P-450 levels as measured by carbon monoxide binding. Although a 24% reduction of cytochrome P-450 was observed in the absence of any measureable 458-nm spectral complex a linear relationship existed between further decreases in the cytochrome and the increase in Type III complex formation (458 nm). Inhibitory potencies of the compounds towards aldrin epoxidase and benzopyrene hydroxylase activities were not clearly correlated with either spectral complex formation or decrease in cytochrome P-450 and it is apparent that different factors are involved in the inhibition of different monooxygenase reactions.     

Effect of some herbicides on CO2 fixation,intermediates pattern,and ruDP-carboxylase and FDPase activities of spinach chloroplasts

We have studied the inhibitory effect of the herbicides phenmediphan, chloroxuron, dinoseb, dichlobenil, dicamba, 2,4-D, 2,4-DB, and 2,4-DP on photosynthetic CO₂ fixation and on the level of intermediates of the CO₂ assimilation cycle by isolated chloroplasts, as well as their in vitro activities on the enzymatic systems ribulose-1,5-diphosphate carboxylase and fructose-1,6-bisphosphatase. Phenmedipham showed the strongest inhibition of CO₂ assimilation, with an I₅₀ of 0.05 μM, followed by chloroxuron and dinoseb, with a 50% inhibition in the range of 0.5–1 μM. A weaker inhibitory effect, with an I₅₀ of 50 μM, is promoted by 2,4-DB, whereas dicamba and 2,4-DP showed this inhibition at 100 μM; dichlobenil and 2,4-D were completely ineffective. In the presence of phenmedipham and chloroxuron, the $\text{trioses-}\text{P}\text{P}\text{-glycerate}$ ratio showed a sharp decrease, which means an inhibition of the P-glycerate reduction step by a low NADPH synthesis; a low ratio is also promoted by 2,4-D, but it may be a consequence of induced collateral metabolic pathways of P-glycerate. Dinoseb showed a 25% inhibition of ribulose-1,5-diphosphate carboxylase activity in the concentration range of 10–100 μM and an I₅₀ of 50 μM of the fructose-1,6-bisphosphatase. Thus these effects could contribute, in addition to the photochemical ones, to an explanation of the dinoseb inhibition of CO₂ assimilation by isolated chloroplasts. The other herbicides tested showed a weak or no effect on these enzyme systems.     

The purification and characterization of esterases from insecticide-resistant and susceptible house flies

Four major esterases in one susceptible (CSMA) and two resistant (Hirokawa, E₁) house fly strains were separated by chromatofocusing. Of the four esterases, those with pI's of 5.1 and 5.3 accounted for 90% of the p-nitrophenyl butyrate hydrolyzing activity in the three house fly strains. They also accounted for 70% (Hirokawa, E₁) and 40% (CSMA) of the paraoxon-hydrolyzing activity as well as 87% (Hirokawa), 39% (E₁) and 66% (CSMA) of the malathion-hydrolyzing activity in microsomes as measured by esterase-antibody interaction. In the Hirokawa strain, the pI 5.1 esterase was the predominant esterase and was more active than that of the the CSMA strain. Different substrate specificities and a different K_m toward acetylthiocholine, as well as different rates of malathion and paraoxon hydrolysis between the Hirokawa and CSMA strains, suggest a qualitative difference in the pI 5.1 esterase. For the pI 5.1 esterase from the E₁ strain, a different substrate specificity, a different K_m for p-nitrophenyl butyrate, a different sensitivity to inhibitors, and a different rate of paraoxon hydrolysis suggest that it is a modified esterase. This esterase is not a phosphorotriester hydrolase, nor does it lack nonspecific esterase activity. It is a modified esterase which has a different substrate specificity when compared to the esterases from the other strains. The molecular weight of the esterases studied was approximately 220,000, with pH optima of about 7.0.The ratio of malathion α-monoacid to β-monoacid formation was about 9.0 for the pI 5.1 and 5.3 esterases and 1.5 for the pI 4.8 and 5.6 esterases. The existence of a higher $\text{α}\text{β}$ ratio for the pI 5.1 and 5.3 esterases and their significant rate of malathion hydrolysis in the Hirokawa strain indicate that an increase in the $\text{α}\text{β}$ ratio in house flies reported was due to the increase in the pI 5.1 esterase in the resistant strain.     

Isomerization and Beckmann rearrangement reactions in the metabolism of methomyl in rats

Methomyl {S-methyl-N-[(methylcarbamoyl)oxy]thioacetimidate}, also known as Lannate, may exist in two geometric configurations but the more stable syn isomer is the form applied as an insecticide. In the rat, syn[¹⁴CN]methomyl [CH₃S(CH₃)CNOC(O)NHCH₃] was metabolized to respiratory ¹⁴CO₂ and $\text{CH}{}_3{}^{14}\text{CN}$ in a ratio of about 2 to 1. Studies with the anti isomer showed that it was metabolized predominately to $\text{CH}{}_3{}^{14}\text{CN}$. These and other data are presented supporting the contention that syn methomyl is partially isomerized to the anti isomer in the animal prior to the hydrolysis of the ester linkage. After hydrolysis, the syn oxime [CH₃S(CH₃)¹⁴CNOH] is further metabolized to ¹⁴CO₂ while the anti oxime is metabolized to $\text{CH}{}_3{}^{14}\text{CN}$. Proposed immediate precursors to the carbon dioxide and acetonitrile, formed by Beckmann rearrangement of the syn and anti oximes, are CH₃S¹⁴C(O)NHCH₃ and $\text{[}\text{CH}{}_3{}^{\mathrm{14\oplus }}\text{CNSCH}{}_3\text{]x}{}^{\mathrm{?}}$, respectively.     

The mechanism of fungistatic action of sec-butylamine: II. The effect of sec-butylamine on pyruvate oxidation by mitochondria of Penicillium digitatum and on the pyruvate dehydrogenase complex

sec-Butylamine at 5 mM inhibited the oxidation of pyruvate by mitochondria isolated from hyphae of Penicillium digitalum, but had little effect on the oxidation of citrate, isocitrate, succinate, malate, acetyl-coenzyme A, or reduced nicotinamide adenine dinucleotide. sec-Butylamine did not interfere with oxidative phosphorylation, as evidenced by similar $\text{P}\text{O}$ ratios in treated and control mitochondria. The pyruvate dehydrogenase complex (EC 1.2.4.1) isolated from young hyphae of P. digitatum was inhibited strongly by 20 mM sec-butylamine, whereas other tricarboxylic acid cycle enzymes were only slightly affected at most. Inhibition of the pyruvate dehydrogenase complex by sec-butylamine was competitive with respect to pyruvate. The K_i for sec-butylamine in the reaction was 1.38 × 10^?2M, and the K_m for pyruvate was 2.28 × 10^?4M. These observations and other evidence derived from studies with intact hyphae support the hypothesis that the pyruvate dehydrogenase complex is the primary site of the fungistatic action of sec-butylamine.     

Cytochrome P-450 content and aldrin epoxidation to dieldrin in isolated rat hepatocytes

A rat hepatocyte suspension effectively epoxidized aldrin to dieldrin with a V_max of 7.19 mol/mol P-450/min and a K_m of 9.27 μM. Viability and metabolic activity were stable for 6 hr after isolation when cells were maintained at room temperature (20°C) with the gentle introduction of $\text{O}{}_2\text{CO}{}_2$ onto the surface of the suspension. The cytochrome P-450 content of the suspension was 303 pmol/10⁶ cells. Primary maintenance culture of the cells also epoxidized aldrin. During culture for 3 days, metabolic activity decreased slowly day by day. Metabolic activity of microsomal fraction from rat liver was also examined. Microsomes epoxidized aldrin with a V_max of 5.11 mol/mol P-450/min and a K_m of 1.64 μM. Significant loss of some subspecies of cytochrome P-450 during fractionation of liver homogenate was indicated.     

Mercapturic acid formation from lindane in rats

4-Chloro-, 2,4-dichloro-, 3,4-dichloro-, 2,3,5-trichloro-, and 2,4,5-trichlorophenylmercapturic acids were identified as main metabolites of lindane, γ-isomer of 1,2,3,4,5,6-hexachloro-cyclohexane, in rat urine. Pathways to these metabolites were shown to include $\text{(}\text{36}\text{45}\text{)-}\text{hexachlorocyclohexene}$ as the most important intermediary metabolite. $\text{(}\text{346}\text{5}\text{)-}\text{Pentachlorocyclohexene}$ and $\text{(}\text{346}\text{5}\text{)-}\text{tetrachlorocyclohexene}$ also seem to be involved in these pathways, while $\text{(}\text{36}\text{45}\text{)-}\text{pentachlorocyclohexene}$ plays a minor role in the pathway. Glutathione conjugation, using the rat liver soluble fraction, occurred directly on the polychlorocyclohexenes, not on their further transformed products. In in vivo biodegradation, $\text{(}\text{36}\text{45}\text{)-}\text{hexachlorocyclohexene}$ may be dechlorinated and dehydrochlorinated at the endoplasmic reticulum before it undergoes the glutathione conjugation in cytosol, although other polychlorocyclohexenes generally react in a manner similar to that in the in vitro reaction.     

Comparative metabolism of six ethoxychlor analogs and DDT in DDT-resistant and susceptible strains of the house fly Musca domestica L

The metabolic fate of six ³H-ring-substituted ethoxychlor analogs with altered aliphatic moieties and [¹⁴C]p,p′-DDT was investigated in susceptible and DDT-resistant strains of the house fly Musca domestica Linnaeus. The chloroalkane analogs, dichloroethane, chloropropane, and dichloropropane were primarily metabolized to the corresponding dehydrochlorinated products. This pathway was relatively more prominent in the resistant strain than in the susceptible strain. Biotransformation and detoxication of the isobutane, nitropropane, and neopentane derivatives was through microsomal oxidation (O-deethylation) of aryl ethoxy degradophores, and oxidation of the aliphatic moieties to produce the corresponding benzophenones, with no substantial differences between the resistant and susceptible strain. There was a strong correlation between the Taft ($\text{σ}{}^1$) values for the altered aliphatic moieties of chloroalkane analogs and their rate of dehydrochlorination in both the strains. These results suggest the importance of altered aliphatic moieties in developing resistance-proof DDT derivatives.     

Deuterium isotope effects on the formation of mercapturic acids from lindane in rats

Metabolism experiments with rats showed that significant isotope effects ($\text{k}{}_{\mathrm{<mtext>H</mtext>}}\text{k}{}_{\mathrm{<mtext>D</mtext>}}\text{= 2.4}\text{to}\text{3.5}$) were associated with the in vivo formation of dichloro and trichlorophenylmercapturic acids from a 1:1 mixture of normal and hexadeuterated lindane. This is evidence that rate-determining dehydrogenation and dehydrochlorination, both of which proceed with significant isotope effects, are essential in the pathway of dichloro- and trichlorophenylmercapturic acid formation from lindane. No significant primary isotope effects were associated ($\text{k}{}_{\mathrm{<mtext>H</mtext>}}\text{k}{}_{\mathrm{<mtext>D</mtext>}}\text{= 1.31 ± 0.17}$) with the formation of monochlorophenylmercapturic acid. This suggests that the 1,2-dechlorination to tetrachlorocyclohexene followed by glutathione conjugation is the probable pathway that produces this metabolite from lindane.     

Optimization of in vitro growth conditions of Pyrenophora teres for production of the phytotoxin aspergillomarasmine A

In liquid cultures of Pyrenophora teres, three phytotoxins may be found: L, L - N -(2-amino-2-carboxyethyl) aspartic acid (toxin A), anhydroaspergillomarasmine A (toxin B) and aspergillomarasmine A (toxin C). In particular, toxins A and C cause chlorotic and necrotic symptoms in detached barley leaves, toxin C being the most damaging, whereas toxin B is only weakly phytotoxic. When P. teres is grown in liquid modified Fries medium, toxin B is the main toxin accumulated, possibly due to a ring closure of toxin C at the low pH value of the medium. The amount of toxin B produced by 11 isolates of P. teres was compared in modified Fries medium. Generally, the most virulent isolates of P. teres produced higher amounts of toxin B than the less virulent isolates. During growth, the pH of the media decreased from 6.7 to about 3.0–3.5, followed by a slight increase to about 3.5–4.0. All isolates, except one, produced toxin B, whereas only two isolates produced toxin C and toxin A. Maintaining the pH at about 6.5 by sterile titration with 1 M NaOH resulted in a shift in toxin accumulation from toxin B to toxin C. The addition of tris or phosphate buffer to the media resulted in higher pH during the growth period, an increase in the total amount of toxins produced, and a shift in toxin accumulation from toxin B to toxin C. The higher pH value probably prevented the conversion of toxin C into toxin B. No toxins were produced in two routinely used media, potato glucose broth and grass broth. Toxin B and toxin C were purified by ion exchange chromatography and precipitation with HCl.     

Induction of the mixed-function oxidase system in the liver of the barn owl Tyto alba by PCBs

Injection of 30 mg/kg body wt of polychlorinated biphenyl (Aroclor 1254) into liver parenchymal tissue of nestling and adult barn owls Tyto alba resulted in increases in the level of cytochrome P-450. Concomitantly, there were increases in catalytic activity of the microsomal enzyme system as measured by aldrin epoxidation and aminopyrine N-demethylation. However, the ratio $\text{455}\text{430}\text{nm}$ in the ethylisocyanide-difference spectrum remained unchanged. Of particular interest is the sudden drop in the level and catalytic activity of cytochrome P-450 in nestling owls at age 40 days. Treatment with Aroclor 1254 produced small hemorrhages in the liver of nestling owls and the liver appeared much enlarged (hepatomegaly), indicating a toxic effect and resulting in little induction of microsomal enzymes. In adult owls the inductive effect was much greater. Aroclor 1254 produced a spectral shift in the cytochrome P-450-difference spectrum from 450 to 448 nm and in the ethylisocyanide-difference spectrum from 455 to 453 nm and from 430 to 427 nm.