Species characteristics of the hepatic xenobiotic and steroid biotransformation systems of two teleost fish, Atlantic cod (Gadus morhua) and rainbow trout (Salmo gairdneri)

Application of a cytochrome P-450 IA1-ELISA in environmental monitoring and toxicological testing of fish

1. An enzyme-linked immunosorbent assay (ELISA) based on polyclonal rabbit anti-cod cytochrome P-450 IA1 IgG has been developed in our laboratory. 2. The antibodies employed in the ELISA demonstrate good cross-reactivity with the homologous protein in a number of other fish species, giving cross-reacting protein bands o 54-59 kDa in liver microsomes, as determined by Western blotting. 3. The ELISA technique has been used in numerous experiments with both field collected and laboratory exposed fish of different species, showing good correlation with contaminant exposure. 4. In some instances of PCB exposure where the classical P-450 IA1 monooxygenase assay 7-ethoxyresorufin O-deethylase (EROD) failed to reveal any induction, the ELISA technique demonstrated increased levels of P-450 IA1 protein, indicating inhibiting effects of the PCBs on EROD measurement. 5. In tissues like gill filaments, and in whole cod larvae, where EROD activity is barely detectable, if at all, the ELISA technique showed induction after exposure to a water soluble fraction (WSF) of North Sea crude oil. 6. The results reviewed indicate the usefulness of the ELISA technique to allow rapid screening of a large number of samples, and especially when catalytic measurement is difficult or impossible due to (a) small sample or tissue size, (b) loss of activities in bad storage conditions, or (c) presence of compounds inhibiting activity.

Cytochrome P450 isozyme distribution in normal and tumor-bearing hepatic tissue from rainbow trout (Salmo gairdneri)

An immunohistochemical technique was used to localize the major constitutive cytochrome P450 isozyme, P450 LM2, and the major beta-naphthoflavone-inducible isozyme, P450 LM4b, in the livers of untreated and aflatoxin B1 (AFB1)-initiated, tumor-bearing rainbow trout. In hepatic tissue sections from untreated trout, no regular anatomical pattern within the hepatic parenchymal cells could be discerned for either isozyme. Immunostaining was observed for P450 LM2 along the sinusoidal border of some of the parenchymal cells, there was moderate staining within the cytoplasm of most cells, and there were focal areas of increased staining. There was intense, uniform immunostaining for P450 LM2 within the cytoplasm of the bile duct cells, in the endothelial lining of arterioles, and along the epithelial surface of the gall bladder. Staining for P450 LM4b in livers from untreated trout was barely detectable. In liver tissue sections from AFB1-treated tumor-bearing fish, P450 LM2 appeared to be reduced and P450 LM4b was absent in the hepatocellular carcinoma nodules. An apparent increase in immunostaining for P450 LM4b was observed in nonneoplastic cells juxtaposed next to neoplastic cells as well as in areas distant to the tumors. These results may indicate that the pattern of P450 isozymes is altered in nonneoplastic cells of tumor-bearing trout livers.

Further consideration of phenobarbital effects on cytochrome P-450 activity in the killifish, Fundulus heteroclitus

1. Ethoxyresorufin O-deethylase (EROD) activity, aldrin epoxidase (AE) activity, cytochrome P-450 content, and levels of cytochrome P-450E (the major BNF-inducible P-450 form and primary EROD catalyst in scup) or its homologues were measured in hepatic microsomes isolated from Fundulus heteroclitus, scup (Stenotomus chrysops) and brook trout (Salvelinus fontinalis) treated with beta-naphthoflavone (BNF) or phenobarbital (PB). 2. In all three teleost species, BNF treatment caused expected increases in P-450 content, EROD activity and P-450E level; but either no change or a slight decrease in AE turnover rate (nmol/min/nmol P-450). 3. Polyclonal antibodies to P-450E did not inhibit AE activity in microsomes from BNF-treated scup, confirming that this major BNF-inducible P-450 form does not catalyze AE activity in fish. 4. In contrast, PB treatment did not affect hepatic AE activity, P-450 content or levels of "P-450E" in F. heteroclitus, but did variably affect EROD activity which was suppressed in one experiment and elevated in another. 5. The results indicate that (i) contrary to previous reports, neither PB nor MC-type inducers increase AE activity in F. heteroclitus, (ii) MC-type inducers do not affect AE activity in the other teleost species examined, and (iii) AE activity is not a reliable indicator of P-450 induction by environmental chemicals. 6. We emphasize the need to establish the mechanism of PB action, and the nature of any fish P-450 forms analogous to PB-inducible forms in mammals in order to conclusively evaluate PB-responses in fish.

Effect of preexposure to dietary benzo[a]pyrene (BP) on the first-pass metabolism of BP by the intestine of toadfish (Opsanus tau): in vivo studies using portal vein-catheterized fish

The effect of preexposure of fish to dietary benzo[a]pyrene (BP) on the intestinal metabolism of BP was examined in toadfish (Opsanus tau). The portal veins of toadfish were cannulated following administration of radiolabeled BP to the intestinal lumen. Because these fish lack a lymphatic vessel system, the portal vein is the sole route by which BP and its metabolites enter the circulation. In fish preexposed to dietary BP (10 mg BP/kg food), the radioactivity entering the portal vein was almost entirely (ca. 90%) BP metabolites. In fish fed a laboratory control diet, a smaller percentage (ca. 60%) of the radioactivity entering the portal vein was in the form of BP metabolites. The enhanced efficiency of the intestines of preexposed fish in metabolizing BP appears to be a result of induction of intestinal aryl hydrocarbon hydroxylase (AHH) activity. Intestinal microsomal AHH activities in control and preexposed fish were 0.033 +/- 0.032 and 0.320 +/- 0.060 nmol.min-1.mg-1, respectively. Gel filtration of portal vein plasma indicated differences in the roles of plasma proteins in transporting BP and BP metabolites. Native BP was associated primarily with the high density lipoproteins, whereas organic-soluble BP metabolites were associated primarily with serum albumin fractions. A large percentage of BP metabolites was recovered as water-soluble conjugates. These studies indicate that in fish, the intestine can be an important organ involved in dietary BP metabolism.