Metabolism of 2,3',4',5-tetrachlorobiphenyl by cytochrome P450 from rats, guinea pigs and hamsters

The metabolism of 2,3',4',5-tetrachlorobiphenyl (TCB) was compared using liver microsomes and six isoforms of cytochrome P450 purified from rats, guinea pigs and hamsters. In microsomal study, the following species differences were observed: 1) Untreated guinea pigs and hamsters but not rats can metabolize this TCB to 3-hydroxy- or 4-hydroxy-2,3',4',5-TCB, 2) Guinea pig microsomes showed only 3-hydroxylating activity, whereas hamster microsomes showed higher activity of 4-hydroxylation than that of 3-hydroxylation. In common with three species, the 3-hydroxylation was accelerated by phenobarbital. The 4-hydroxylation in rats and hamsters was increased by pretreatment with 3-methylcholanthrene and 3,3',4,4',5-pentachlorobiphenyl. The hydroxylation activities of liver microsomes from the three species could be explained by an involvement of different isoforms of cytochrome P450. In addition, it is apparent that hamster CYP1A2 as well as hamster CYP2A8 is involved in the 4-hydroxylation of 2,3',4',5-TCB although it has no activity for 2,2',5,5'-TCB.

Highly toxic coplanar PCB126 reduces liver peroxisomal enzyme activities in rats

The effect of the highly toxic coplanar PCB congener, 3,4,5,3',4'-pentachlorobiphenyl (PCB126) on hepatic peroxisomes was studied in rats. The aim of this study was to investigate whether a toxic dose of the dioxin-like coplanar PCB modifies enzyme activities in peroxisomes where plays an important role in lipid metabolism. Treatment with PCB126, at a single i.p. administration of 25 mg/kg which evokes clear suppression of body weight gain, resulted in marked reduction (to about 40-50%) of catalase activity and peroxisomal fatty acyl-CoA β-oxidizing system. Immunoblotting showed that expression of catalase was greatly reduced by the treatment in parallel with the activity. Light microscopy revealed a drastic reduction in granules possessing peroxidase activity, while electron microscopy demonstrated that no apparent morphological changes had taken place. Thus the reduction in catalase activity caused by PCB126 could be attributable to suppression of protein expression. The marked reduction of these peroxisomal enzyme activities might be related to hyperlipidemia caused by dioxin-related compounds in rats and humans.

Characterization of the rat Star gene that encodes the predominant 3.5-kilobase pair mRNA. ACTH stimulation of adrenal steroids in vivo precedes elevation of Star mRNA and protein

The steroidogenic acute regulatory protein (STAR) participates in steroidogenesis through the mitochondrial transfer of cholesterol to cytochrome P450scc. The rat adrenal Star gene is transcribed as a 3. 5-kilobase pair (kb) and 1.6-kb mRNA with the larger mRNA predominating ( approximately 85% of total) in vivo. Hypophysectomy (HPX) produced a 3-5-fold decrease in Star mRNA along with a loss of adrenal steroids, whereas P450scc mRNA decreased by less than 2-fold. Adrenocorticotropic hormone (ACTH) treatment of HPX rats maximally stimulated steroidogenesis rates within 5 min with over 10-fold elevation of steady state blood levels occurring within 10 min. For intact rats there was a 5-10-fold larger increase, paralleling previously observed elevations of cholesterol-cytochrome P450scc association and metabolism in subsequently isolated adrenal mitochondria. ACTH did not increase either total STAR protein or a group of modified forms until at least 30 min after completion of acute stimulation, indicating that elevated translation of STAR protein cannot alone mediate this acute stimulation. Parallel slow changes in STAR protein and corticosterone formation after ACTH treatment are consistent with participation of STAR forms as co-regulators of these hormonal responses. ACTH stimulation of HPX rats increased Star mRNA by 2.5-fold within 20 min and by 4.5-fold after 1 h, thus preceding the rise in the STAR protein. A 3.5-kb Star cDNA clone isolated from a rat adrenal cDNA library exhibited a 0.9-kb open reading frame and a 2.5-kb 3'-untranslated region (3'-UTR). The open reading frame sequence differed at only 12 amino acids from that of the mouse Star. The rat Star gene seven exons with exon 7 encoding the entire 2.5 kb of 3'-UTR of the 3.5-kb mRNA. The 3'-UTR sequence suggests that 1.6- and 3.5-kb mRNA are formed by an alternative usage of different polyadenylation signals. Multiple UUAUUUA(U/A)(U/A) motifs also suggest additional regulation through this extended 3'-UTR. Although elevation of STAR protein by ACTH does not cause the acute increase in adrenal cholesterol metabolism, changes in the turnover or distribution of an active STAR subfraction cannot be excluded.

Metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl (PCB153) in guinea pig

1. The in vitro and in vivo metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl (PCB153) in guinea pig has been studied. 2. Seven metabolites were detected in the faeces of PCB153-treated animals and three were identical to those produced by dog liver microsomes. The detection of a metabolite where a chlorine atom was shifted from the 2- to 3-position strongly suggested the involvement of 2,3-arene oxide intermediate, and evidence for the concomitant formation of a 3,4-arene oxide intermediate was provided by identifying other two minor metabolites which were dechlorinated at the 4-position. 3. In vitro studies using liver microsomes from guinea pigs revealed that the 2,3-arene oxide and 3-hydroxylation pathways are the predominant metabolic routes compared with the 3,4-arene oxide pathway. Although the guinea pig is an another species that can metabolize PCB153 mainly to the 2,3-arene oxide intermediate, the rate of formation was only about one-tenth of the dog. 4. These results indicate that the ability to form this unusual 2,3-arene oxide intermediate may not be responsible for high excretion rate of this congener. Our data also suggest that the cytochrome P450-catalysed metabolism of PCB153 in the guinea pig and dog are similar, whereas for post-cytochrome P450 metabolism, the guinea pig resembles the rabbit.

Species-specific alteration of hepatic glucose 6-phosphate dehydrogenase activity with coplanar polychlorinated biphenyl: evidence for an Ah-receptor-linked mechanism

We examined the in vivo effect of a highly toxic coplanar polychlorinated biphenyl (PCB) on the hepatic activity of glucose 6-phosphate dehydrogenase (G6PDH) in aryl hydrocarbon (Ah)-responsive (C57/BL) and -less-responsive (DBA) strains of mice. The activity in the C57BL strain was moderately increased by 3,3',4,4',5-pentachlorobiphenyl (PCB 126) in a dose dependent manner. However, this was not observed in DBA mice although greater doses were injected. 2,2',5,5'-Tetrachlorobiphenyl (PCB 52) with a non-planar structure did not increase G6PDH activity. The increase in G6PDH activity with PCB 126 was also seen in rats, but not in guinea pigs. The activity in the latter species was decreased rather than increased. These results suggest that the induction of hepatic G6PDH by coplanar PCB is mediated by a mechanism involving the Ah receptor, and the response was highly species-specific.

Changes in the hepatic glutathione peroxidase redox system produced by coplanar polychlorinated biphenyls in Ah-responsive and -less-responsive strains of mice: mechanism and implications for toxicity

The alteration in hepatic glutathione peroxidase (GPx) produced by polychlorinated biphenyls (PCBs) was studied in vivo in aryl hydrocarbon (Ah)-responsive C57BL and -less-responsive DBA strains of mice. 3,3',4,4',5-Pentachlorobiphenyl (PCB 126), one of the high-affinity ligands for the Ah receptor, significantly reduced Se-dependent GPx activity in C57BL mice, but not in DBA mice. A reduction in activity in C57BL mice was also observed following treatment with a high dose of 3,3',4,4'-tetrachlorobiphenyl with lesser affinity for the Ah receptor than PCB 126, but not by 2,2',5,5'-tetrachlorobiphenyl, a low-affinity ligand. To assess the effects on GPx in the liver, the content of reduced glutathione (GSH), an obligate co-factor for GPx, and the activity of two enzymes, γ-glutamyl transpeptidase (γ-GTP) and glutathione reductase (GR), which play a role in supplying GSH were determined after PCB treatment. The results showed that although the hepatic activity of γ-GTP and GR was affected differently by PCB 126, the content of GSH was slightly increased rather than reduced in both strains of mice. The activity of non-Se-dependent GPx, which is due to the catalysis by some isozymes of glutathione S-transferase (GST), was significantly increased only in C57BL mice by PCB 126 treatment. Immunoblot analysis demonstrated that the induction of the class θ GST, which is a potent reducer of peroxides (Hiratsuka et al., 1995. Biochem. Biophys. Res. Commun. 212, 743) reflects the enhancement of the above activity. These results suggest that (i) the PCB-induced reduction in Se-dependent GPx activity is mediated by a mechanism involving the Ah receptor; and (ii) a concomitant increase in the class θ GST partially rescues the Ah-responsive mice from coplanar PCB-induced oxidative stress.

[Production of antibody against cytosolic 54 kDa protein in rat liver--evidence of the significant induction by a highly toxic coplanar polychlorinated biphenyl]

We have reported that a 54 kDa protein in rat liver cytosol is highly inducible by treatment with 3,3',4,4',5-pentachlorobiphenyl (PenCB) or 3-methylcholanthrene (MC) using SDS-polyacrylamide gel electrophoresis. Internal amino acid sequences of this protein in the rat liver were highly homologous to those of selenium binding protein (SeBP) or acetaminophen binding protein (APBP) in mouse liver cytosol. In this paper, the purification and characterization of this protein were demonstrated. MC was given at a dose of 20 mg/kg for 3 consecutive days. The liver cytosolic 54 kDa protein was purified twice from the MC-treated male Wistar rats by Rotofore Cell procedure to apparent single on SDS-polyacrylamide gel electrophoresis, and the rabbit antiserum against this protein was obtained. Male Wistar rats were given PenCB in corn oil at a single dose of 25 mg/kg i.p. The liver cytosol was prepared on the 5th day after the treatment and subjected to immunoblot analysis. The 54 kDa protein was markedly induced in the liver cytosol of PenCB-treated rats. Immunoblot analysis after two-dimensional gel electrophoresis suggested that there could be isoforms of 54 kDa protein. The induction of the 54 kDa protein with PenCB was assumed to be mediated through Ah-receptor. The physiological role of the 54 kDa protein was discussed together with SeBP and APBP, the role of which has not yet been elucidated.

[Effect of co-planar polychlorinated biphenyl on the hepatic glutathione peroxidase redox system in rats and guinea pigs]

The effect of 3,3',4,4',5-pentachlorobiphenyl (PCB 126) on hepatic glutathione peroxidase (GPx) redox system was studied in vivo in rats and guinea pigs. PCB 126 treatment caused significant reduction of Se-dependent and -non-dependent GPx activity in rats. In agreement with this, the content of glutathione (GSH) and the activities of GSH reductase (GR) and gamma-glutamyl transpeptidase (gamma-GTP) were also decreased in this species. On the contrary, guinea pig liver Se-non-dependent GPx activity was significantly enhanced by PCB 126 treatment, while no effect on Se-dependent activity was observed. Neither the content of GSH nor the enzyme activities responsible for GSH supply in guinea pig liver was affected by PCB 126. These result suggested that the damage on GPx redox system is, at least, one of mechanisms by which co-planar PCB induces the toxicity in rats. However, in guinea pigs, this is not the case, and different mechanism from the damage on active oxygen quenching system is likely to be involved.

A highly toxic PCB produces unusual changes in the fatty acid composition of rat liver

The changes in lipid metabolism produced by a coplanar PCB were studied in rats. Male Wistar rats were given a single intraperitoneal injection of 3,3',4,4',5-pentachlorobiphenyl at a dose of 25 mg/kg. After 5 days of administration, total hepatic lipids were treated with 1 M KOH in methanol at 75 degrees C and the liberated fatty acids were analyzed by HPLC after conversion to fluorescent derivatives. In comparison with free-fed and pair-fed control groups, the proportion of arachidonic acid in the PenCB-treated rats was reduced by about 50%, while oleic and linoleic acids increased significantly. We also examined the individual glycerophospholipids, separated by TLC, to see if they were affected by alteration in the fatty acid composition of the whole liver. In all glycerophospholipids, the proportion of arachidonic acid was reduced significantly to the same degree while linoleic acid increased. Changes in the activity of desaturase isozymes have been postulated to explain this unusual lipid metabolism following administration of a toxic PCB and this may contribute to its toxicity.

Significant suppression of rat liver aldolase B by a toxic coplanar polychlorinated biphenyl, 3,3',4,4',5-pentachlorobiphenyl

A toxic coplanar polychlorinated biphenyl, 3,3',4,4',5-pentachlorobiphenyl (PenCB), significantly suppresses the expression of liver aldolase B in rats. Hepatic aldolase activity in PenCB-treated rats was significantly reduced to about 50% of that in free- and pair-fed control groups. The reduced aldolase activity following PenCB-treatment was due to the marked suppression of the expression of aldolase B shown by immunoblot analysis after SDS-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis. The suppression of rat liver aldolase B could be a key biochemical lesion caused by PenCB.

Control of cholesterol access to cytochrome P450scc in rat adrenal cells mediated by regulation of the steroidogenic acute regulatory protein

Cholesterol conversion to pregnenolone by cytochrome P450scc in steroidogenic cells, including those of the adrenal cortex, is determined by hormonal control of cholesterol availability. Intramitochondrial cholesterol movement to P450scc, which retains hormonal activation in isolated mitochondria, is apparently dependent on peripheral benzodiazepine receptor and the recently cloned steroidogenic acute regulatory (StAR) protein. In rat adrenal cells, StAR is formed as a 37-kDa precursor that is transferred to the mitochondrial inner membrane following phosphorylation by hormonally activated protein kinase A, and processed to multiple forms, some of which turn over very rapidly. In bovine cells, StAR undergoes three modifications forming a set of eight proteins seen in both glomerulosa and fasciculata cells. In the former, cyclic AMP and angiotensin II each decrease two forms and elevate six forms. Significantly, the major change seen after activation may not involve phosphorylation of StAR. Cholesterol transfer across mitochondrial membranes is also activated in isolated mitochondria by GTP and low concentrations of Ca2+, apparently prior to activation by StAR. Depletion of StAR by cycloheximide inhibits cholesterol transfer but is overcome by uptake of Ca2+ into the matrix. This activation of cellular cholesterol transport is sustained in adrenal cells permeabilized by Streptolysin O. In rat adrenal cells cAMP elevates 3.5- and 1.6-kb mRNA, hybridized by a 1.0-kb StAR cDNA. A 3.5-kb rat adrenal cDNA that encodes all except the 5' end of the longest StAR mRNA has been characterized. The corresponding gene sequence is distributed across seven exons. The shorter mRNA may arise from polyadenylation signals early in exon 7. However, the 3.5-kb mRNA comprises 80-90% of untreated rat adrenal StAR mRNA and may therefore provide the prime source for in vivo translation of StAR protein.

Overview of clinical studies of childhood acute lymphoblastic leukemia for more than ten years by the Japanese Children's Cancer and Leukemia Study Group

Since 1981, the Children's Cancer and Leukemia Study Group (CCLSG) has developed a series of protocols for treatment of acute lymphoblastic leukemia (ALL) in childhood. In the first randomized controlled study of the 811 protocol (1981-1983) a comparison of conventional daily 6-mercaptopurine and methotrexate with a pulsed regimen of the two drugs was performed. The superiority of the pulsed regimen was shown. In the next 841 protocol (1984-1987) a comparison of two drugs and three drugs during induction therapy was conducted. The three-drug regimen resulted in a significantly higher event-free survival (EFS) rate. In the 874 protocol (1987-1990) two regimens with or without cranial irradiation were randomly compared, and there was no significant difference between the two regimens for the standard-risk group. To further improve the EFS rate a risk group-directed protocol 911 was conducted starting in January 1991. Life-table analysis of serial CCLSG protocols revealed that the outcome of overall ALL has gradually improved with an increase of the EFS rate; 41.4% +/- 3.6% at 14 years for the 811 protocol, 51.3% +/- 3.5% at 11 years for the 841 protocol, 56.7% +/- 3.1% at 8 years for the 874 protocol, and 78.2% +/- 3.1% at 4 years for the more recent 911 protocol.

Significant induction of a 54-kDa protein in rat liver with homologous alignment to mouse selenium binding protein by a coplanar polychlorinated biphenyl, 3,4,5,3',4'-pentachlorobiphenyl and 3-methylcholanthrene

A 54-kDa protein in rat liver cytosol was significantly induced by treatment with 3,4,5,3',4'-pentachlorobiphenyl (25 mg/kg, single i.p.) and 3-methylcholanthrene (20 mg/kg, once a day for 3 days, i.p.). The protein exhibited pI of 6.8 on two-dimensional gel electrophoresis. The amino acid sequences of peptide fragments from the protein digested in situ were highly similar to a selenium binding protein in mice and to the isoform acetaminophen binding protein in mice. The present result clearly demonstrates that a coplanar polychlorinated biphenyl and 3-methylcholanthrene are responsible for induction of selenium binding protein homologues. The physiological role of the mouse proteins, however, is not yet elucidated.

Hamster liver cytochrome P450 (CYP2A8) as a 4-hydroxylase for 2,5,2',5'-tetrachlorobiphenyl

Metabolism of 2,5,2',5'-tetrachlorobiphenyl (TCB) was studied using liver microsomes of hamsters and two hamster P450 isoforms, CYP1A2 and 2A8. CYP2A8 catalyzed selectively 4-hydroxylation of 2,5,2',5-TCB at a rate of 21.7 pmol/min/nmol P450. In contrast, CYP1A2 showed no activity for hydroxylation of 2,5,2',5'-TCB. Immunological study revealed that rabbit antiserum against CYP2A8 almost completely inhibited the microsomal 4-hydroxylation but that against CYP1A2 did not. It was also shown that the induction pattern of CYP2A8 protein by P450 inducer was similar to that of the 4-hydroxylase activity in hamster liver microsomes. These results suggest that CYP2A8 plays a major role in the 4-hydroxylation of 2,5,2',5'-TCB in hamster liver.

Role of cytochrome b5 in the oxidative metabolism of polychlorinated biphenyls catalyzed by cytochrome P450

1. The role of cytochrome b5 in the cytochrome P450-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted system consisting of CYP2B1 and CYP1A1 and rat liver microsomes. 2. By addition of cytochrome b5 to the reconstituted system containing CYP2B1, the 3-hydroxylation of 2,5,2,'5'- and 2,5,3',4'-TCB was increased about six-fold, but the 3- and 5-hydroxylation of 2,4,3',4'-TCB was decreased by about 50% 3. All hydroxylations of 3 ,4,3',4'-,2,5, 3,4'- and 2,4,3',4'-TCBs were decreased by addition of cytochrome b5 to the reconstituted system containing CYPlA1. 4. In stoichiometry measurements, changes in NADPH oxidation and coupling efficiency by addition of cytochrome b5 was observed and these differed according to the position of chlorine atoms of TCBs and cytochrome P450 isoforms used in the systems.

A coplanar PCB induces a selenium binding protein as a major cytosolic protein in rat liver

We obtained evidence that a toxic coplanar polychlorinated biphenyl (PCB) induces a counterpart of murine 56kDa selenium binding protein in rat liver cytosol. A 54kDa protein in the liver cytosol was significantly induced by 3,3',4,4',5-pentachlorobiphenyl and proved to be a major cytosolic protein in the rat liver. The protein exhibited pI of 6.8 on two-dimensional gel electrophoresis. The amino acid sequence of peptide fragments from the protein digested in situ, was highly similar to a 56kDa selenium binding protein and similar to an acetaminophen binding protein in mice.

Involvement of cytochrome b5 in the metabolism of tetrachlorobiphenyls catalyzed by CYP2B1 and CYP1A1

The role of cytochrome b5 in the cytochrome P450 (CYP)-dependent hydroxylation of tetrachlorobiphenyl (TCB) isomers was examined using a reconstituted mixed function oxygenase (MFO) system containing purified CYP2B1 or 1A1, and rat liver microsomes. Hydroxylations of 2,2',5,5'- and 3,3',4,4'-TCBs were catalyzed mainly by CYP2B1 and 1A1, respectively, in the reconstituted MFO system and those of 2,3',4',5- and 2,3',4,4'-TCBs were mediated by both cytochrome P450 systems. The activity toward 2,2',5,5'- and 2,3',4',5-TCB was significantly increased 6.5- and 5.5-fold, respectively, by addition of cytochrome b5 in the reconstituted MFO system containing of CYP2B1. Either hydroxylation activity toward 2,3',4,4'-TCB with the CYP2B1 system was very low or decreased by addition of cytochrome b5. These results suggest that the involvement of cytochrome b5 to the hydroxylation of TCBs is dependent on the TCB congener being metabolized, and the cytochrome P450 isoform involved in its metabolism.

Structure of genes for sperm-specific nuclear basic protein (SP4) in Xenopus laevis

Nuclear basic proteins in sperm of Xenopus laevis consist of 6 sperm-specific proteins (SPs1-6) in addition to somatic core histones. Using a cDNA for SP4 as a probe, we cloned genomic DNA containing SP4 genes from a genomic library constructed from recombinant lambda bacteriophage containing 12.0 kbp-EcoRI digests of J-strain X. laevis liver DNA. Construction of restriction maps based on Southern blot analysis revealed the existence of a total of five SP4 genes which are arranged in a tandemly repeated array forming a cluster of simple multigenes per haploid genome, over a range of 18 kbp. Among these genes, the one located at the most upstream position differed from others in possessing a single base substitution which gave rise to a replacement of one out of 78 amino acid residues. The DNA containing the second to the fourth SP4 genes, arranged at about 3 kbp intervals each, was totally sequenced for 10,165 bp. Each gene was found to contain one intron, typical TATA and CCAAT boxes in the 5'-flanking region, and a polyadenylation signal in the 3'-flanking region. Comparative sequence analyses revealed three regions of extensive homology within the upstream non-coding region among three genes, suggesting a possible relevance to their expression at a particular phase of spermatogenesis and/or in testis.

[Hand-mirror cells acute lymphoblastic leukemia (L3)]

To our knowledge, this report represents the first confirmed case in Japan of a 15-year-old boy with acute lymphoblastic leukemia (ALL).L3 with hand-mirror cells (HMC) in the bone marrow. HMC lymphoid leukemia is an unusual variant of ALL in which the bone marrow lymphoblasts manifest distinctive hand-mirror morphologic features. HMC lymphoblast is characterized by an asymmetric foot-like cytoplasmic process that extends from the portion of the cell, thus giving it the light-microscopic appearance responsible for its name. Besides ALL, HMC has been reported in acute myeloblastic leukemia (AML), blastic crisis of chronic myelogenous leukemia, non-Hodgkin's lymphoma, and infectious mononucleosis. HMC has been reported to be prevalent in ALL.L1 and L2 as compared with L3.

Metabolism of highly persistent PCB congener, 2,4,5,2',4',5'-hexachlorobiphenyl, by human CYP2B6

Metabolism of 2,4,5,2',4',5'-hexachlorobiphenyl was studied with cDNA-expressed human P450 2B isoform, CYP2B6. 3-Hydroxy-2,4,5,2',4',5'-hexachlorobiphenyl was identified as a major metabolite, and the formation activity was compared with that of dog CYP2B11 and guinea pig P450GP-1. The activity of 3-hydroxylation was comparable with that of P450GP-1, but one-tenth of CYP2B11. These results indicate that P450 2B in humans as well as other animal species can metabolize 2,4,5,2',4',5'-hexachlorobiphenyl, and the reason why this PCB congener remained most abundantly in human bodies is discussed.

[Alteration of peroxisomal enzyme activities in the liver of guinea pigs caused by coplanar PCB]

The hyperlipidemia is a well-known typical symptom in Yusho patients and experimental animals treated with PCBs. We have found a significant induction of CYP4A1, which catalyzes omega-hydroxylation of fatty acids, in guinea pigs by the treatment with a coplanar PCB, 3, 4, 5, 3',4-pentachlorobiphenyl (PenCB), though the P450 is reduced in the treated rats. Peroxisome has beta-oxidation enzymes distinct from mitochondrial enzymes, and also play an important role in lipid metabolism. Peroxisome proliferators have been shown to regulate the expression of CYP4A1 and peroxisomal enzymes by the same mechanism in the rat. In the present study, we examine the effect of PenCB treatment on peroxisomal enzymes in the liver of guinea pigs. As a result, the enzyme activities of hepatic peroxisome, e.g. fatty acid oxidizing system, catalase and urate oxidase, had a rising tendency by the treatment with PenCB in the animal. The results suggest that the regulation of peroxisomal enzymes and CYP4A1 is also associated in guinea pigs, and PenCB provides a similar effect of peroxisomal proliferators to the animal. The possible toxicity through the peroxisomal alteration was discussed.

[Effect of a coplanar PCB on lipid metabolism: the remarkable difference between rats and guinea pigs]

We studied the effect of 3, 4, 5, 3', 4'-pentachlorobiphenyl (PenCB) on lipid metabolism by determining the level of triacylglycerol and total cholesterol in rats and guinea pigs. Male Wistar rats and male Hartley guinea pigs received PenCB in corn oil one at a dose of 25 mg/kg i.p. and 0.5 mg/kg i.p., respectively. Pair-fed control group of both species were treated with the vehicle and given the amount of chow matched with that taken by the PenCB-treated animals. Free-fed control group was treated with vehicle and was given the chow ad libitum. The plasma was collected on the day 5 after the treatment and the liver was removed. The plasma triacylglycerol level in guinea pigs treated with PenCB was significantly higher than those in free- and pair-fed controls, whereas no significant difference was observed in PenCB-treated rats from both control groups. The plasma cholesterol level was also higher in PenCB-treated guinea pigs than in the two control groups, though the level in rats was significantly lower than the corresponding control values. The hepatic triacylglycerol and cholesterol levels were increased significantly in both species by the PenCB treatment. Although lipid metabolism was disordered in both animals by treatment with PenCB, the responsiveness was remarkably different between guinea pigs and rats. These differences could be associated with species difference in susceptibility toward toxic chlorinated aromatic hydrocarbons.

[Studies on PCB toxicity involving 2C subfamily cytochrome P450]

It has been demonstrated that PCB metabolism is mainly catalyzed by 1A and 2B subfamily cytochrome P450s, CYP1A1/2 and CYP2B1/2. These studies were conducted mostly with hepatic enzymes in rodents. The 1A and 2B subfamily P450 s are constitutively expressed little, but markedly induced by xenobiotics such as 3-methylcholanthrene and phenobarbital in rodents. On the other hand, the recent studies showed that cytochrome P450s in human liver are remarkably different from isoform of rodents in constitution and enzyme activities. In the present study, we first tried to metabolize some PCBs with 2C subfamily cytochrome P450 (CYP2C) purified from dog liver microsomes. The data suggested that CYP2C may not be involved in PCB metabolism. Since CYP2C is the same most abundant enzyme as 3A subfamily P450 in human liver and plays a major role for metabolism of many drugs used clinically, and may also play an important role for metabolism of some steroid hormones, we further studied the inhibition of CYP2C-catalyzed steroid metabolism by typical PCB congeners. CYP2C-mediated steroid metabolism is greatly inhibited by 2, 4, 5, 2', 4', 5'-hexachlorbiphenyl, but not by 3, 4, 5, 3', 4'-pentachlorobiphenyl. On the contrary, 3, 4, 5, 3', 4'-pentachlorobiphenyl markedly suppressed CYP2C expression in the dog liver. These results suggest that residual PCBs may affect the current situation of steroid hormones in Yusho patients, and may cause PCB drug interactions.

Assessment of testicular biopsy after cessation of maintenance chemotherapy in childhood acute lymphoblastic leukemia: a report from the Children's Cancer and Leukemia Study Group

Among 484 male patients with acute lymphoblastic leukemia (ALL) registered into the protocols CCLSG 811, 841 and 874, from 1981 through 1990, 246 patients completed their protocols and were in continuous complete remission (CCR) for more than 3 years. One hundred and seven patients received bilateral testicular biopsies at the time of cessation of maintenance chemotherapy. Eight patients (7.5%) were found to have occult testicular leukemia (TL). Three of them did not receive any additional therapy and all suffered subsequent relapses; one bone marrow relapse and two testicular relapses. The other 3 patients received testicular radiation combined with an additional 2 years of chemotherapy, resulting in CCR for more than 6 years 10 months, 7 years 6 months, and 8 years 6 months. One with chemotherapy alone and another with radiation alone showed subsequent relapse. Overt TL after negative initial biopsy was developed in 3 (3.0%) of the 99 patients. All of them received testicular radiation with chemotherapy, resulting again in CCR for more than 1 year 0 months and 5 years 3 months; one patient showed relapse in testes and bone marrow after 3 years 8 months of CCR. These studies suggested that occult TL has an adverse prognostic significance unless retrieval chemotherapy is given and that performance of testicular biopsy at completion of maintenance chemotherapy is not contributory to prolongation of disease-free survival for males with ALL because the treatment employing testicular radiation plus retrieval chemotherapy for both occult TL and isolated overt TL after off-therapy is similarly very effective.