An overview of benzene metabolism.

Benzene toxicity involves both bone marrow depression and leukemogenesis caused by damage to multiple classes of hematopoietic cells and a variety of hematopoietic cell functions. Study of the relationship between the metabolism and toxicity of benzene indicates that several metabolites of benzene play significant roles in generating benzene toxicity. Benzene is metabolized, primarily in the liver, to a variety of hydroxylated and ring-opened products that are transported to the bone marrow where subsequent secondary metabolism occurs. Two potential mechanisms by which benzene metabolites may damage cellular macromolecules to induce toxicity include the covalent binding of reactive metabolites of benzene and the capacity of benzene metabolites to induce oxidative damage. Although the relative contributions of each of these mechanisms to toxicity remains unestablished, it is clear that different mechanisms contribute to the toxicities associated with different metabolites. As a corollary, it is unlikely that benzene toxicity can be described as the result of the interaction of a single metabolite with a single biological target. Continued investigation of the metabolism of benzene and its metabolites will allow us to determine the specific combination of metabolites as well as the biological target(s) involved in toxicity and will ultimately lead to our understanding of the relationship between the production of benzene metabolites and bone marrow toxicity.

Metabolism of [14C]phenol in the isolated perfused mouse liver.

A previous report from this laboratory focused on the metabolism of [14C]benzene (BZ) in the isolated, perfused, mouse liver (C. C. Hedli, et al., 1997, Toxicol. Appl. Pharmacol. 146, 60-68). Whereas administration of BZ to mice results in bone marrow depression (R. Snyder et al., 1993, Res. Commun. Chem. Pathol. Pharmacol. 20, 191-194), administration of phenol (P), the major metabolite of BZ, does not. It was, therefore, of interest to determine whether the metabolic fate of P produced during BZ metabolism differed from that of P metabolized in the absence of BZ. Mouse livers were perfused with a solution of [14C]P in both the orthograde (portal vein to central vein) and retrograde (central vein to portal vein) direction to investigate the metabolic zonation of enzymes involved in P hydroxylation and conjugation. Perfusate samples were collected, separated by HPLC, and tested for radioactivity. Unconjugated metabolites were identified by comparing their retention times with nonradiolabeled standards, which were detected by UV absorption. Conjugated metabolites were identified and collected on the basis of radiochromatogram results, hydrolyzed enzymatically, and identified by co-chromatography with unlabeled BZ metabolites. The objective was to compare and quantify the metabolites formed during the perfusion of P in the orthograde and retrograde directions and to compare the orthograde P-perfusion results with the orthograde BZ results reported previously. Regardless of the direction of P perfusion, the major compounds released from the liver were P. phenylgucuronide, phenylsulfate, hydroquinone (HQ), and HQ glucuronide. A comparison of the results of perfusing P in the orthograde versus the retrograde direction showed that more P was recovered unchanged and more HQ was formed during retrograde perfusion. The results suggest that enzymes involved in P hydroxylation are generally closer to the central vein than those involved in conjugation, and that during retrograde perfusion, P metabolism may be limited by the sub-optimal conditions of perfusion. Comparison of the orthograde perfusion studies of P and BZ revealed that a larger percentage of the radioactivity released from the liver was identified as unconjugated HQ after BZ perfusion than after P perfusion. In addition, the amount of radioactivity covalently bound to liver macromolecules was measured after each perfusion and determined to be proportional to the amount of HQ and HQG detected in the perfusate samples.

Effects of benzene metabolite treatment on granulocytic differentiation and DNA adduct formation in HL-60 cells.

Reactive metabolites of benzene (BZ) play important roles in BZ-induced hematotoxicity. Although reactive metabolites of BZ covalently bind to DNA, the significance of DNA adduct formation in the mechanism of BZ toxicity is not clear. These studies investigated the covalent binding of the BZ metabolites hydroquinone(HQ) and 1,2,4-benzenetriol(BT) using the DNA [32P]postlabeling method and explored the potential relationship between DNA adduct formation and cell differentiation in human promyelocytic leukemia (HL-60) cells, a model system for studying hematopoiesis. Maturation of HL-60 cells to granulocytes, as assessed by light and electron microscopy, was significantly inhibited in cells that were pretreated with HQ or BT prior to inducing differentiation with retinoic acid (RA). The capacity of RA-induced cells to phagocytose sheep red blood cells (RBC) and to reduce nitroblue tetrazolium (NBT), two functional parameters characteristic of mature, differentiated neutrophils, was also inhibited in cells pretreated with HQ or BT. These BZ metabolite treatments induced DNA adduct formation in HQ- but not in BT-treated cells. These results indicate that whereas HQ and BT each block granulocytic differentiation in HL-60 cells, DNA adducts were observed only following HQ treatment. Thus DNA adduct formation may be important in HQ but not in BT toxicity.

Benzene metabolism in the isolated perfused mouse liver.

The hematotoxicity of benzene (BZ) requires its hepatic metabolism, the release of metabolites into the circulation, and the access of metabolites to the bone marrow. Although a range of potentially toxic metabolites produced by the liver was identified using subcellular systems and isolated hepatocytes, these models do not allow identification of the metabolites released from the liver with respect to time and flow through the liver. We developed an isolated perfused mouse liver model to evaluate metabolites released following a single-pass of radiolabeled BZ and after recirculation of single-pass metabolites back through the liver. Reversing the path of flow through the liver changes the orientation of hepatic oxidizing and conjugating enzymes with respect to perfusate flow. Comparison of metabolite production following normal (orthograde, portal vein to hepatic vein) perfusion with reversed (retrograde) perfusion permitted an evaluation of the impact of zonal distributions of these enzymes on BZ metabolism. The major metabolites detected by HPLC, irrespective of the direction of perfusion, were free phenol (P), phenylsulfate (PS), and phenylglucuronide (PG), plus lesser amounts of hydroquinone (HQ) and hydroquinone glucuronide (HQG). Recirculation of the products of single pass orthograde perfusion through the liver yielded P conjugates as well as low levels of free and conjugated HQ. No free P was detected after recirculation. Although no qualitative differences between orthograde and retrograde perfusion were observed, the percentage of free P and P conjugates (PS + PG) found as free P was twice as great following orthograde perfusion as compared to retrograde perfusion. These results suggest that regional differences in the zonation of enzymes involved in oxidation and conjugation may play a critical role in hepatic BZ metabolism.

Investigation of hepatic cytochrome P-450 enzyme induction and DNA adduct formation in male CD/1 mice following oral administration of toxaphene.

Exposure of experimental animals to toxaphene induces hepatic cytochrome P-450 (CYP). Although chronic administration of toxaphene to mice was found to cause an increased incidence of liver tumors, a mechanism for its carcinogenicity has yet to be elucidated. We investigated two potential mechanisms of toxaphene-induced carcinogenicity: peroxisomal proliferation and DNA binding. Peroxisomal proliferation was evaluated by measuring the level of immunodetectable CYP 4A1, an isozyme of CYP that is specifically induced by peroxisomal proliferators, in hepatic microsomes from CD1 mice that were treated by oral gavage for seven consecutive days with corn oil vehicle or 10, 25, 50 or 100 mg kg(-1) toxaphene. In comparison to control mice, toxaphene-treated mice had increased liver weight, increased liver/body weight ratios and increased levels of total hepatic CYP and cytochrome b5. No increase in the level of immunodetectable levels of CYP 4A1 was found in hepatic microsomes from toxaphene-treated mice when compared to controls. In contrast, increases in immunodetectable CYP 4A1 were detected in hepatic microsomes from mice treated with the peroxisomal proliferator clofibrate. These findings suggest that toxaphene-induced induction of CYP may not involve CYP 4A1 and that peroxisomal proliferation may not be involved in toxicity. Significant increases in immunodetectable levels of CYP 2B were, however, detected in toxaphene-treated mice, and are consistent with earlier reports demonstrating that toxaphene, like many other pesticides, induces the phenobarbital-inducible subfamily of CYP. Analysis of DNA adduct levels in the livers of toxaphene-treated mice by DNA 32P-post-labeling showed no evidence of DNA adduct formation.

Evaluation of calcium entry blockers in several models of immediate hypersensitivity.

Several calcium-entry blockers, i.e., verapamil, nifedipine, flunarizine and diltiazem, were evaluated for their effects in models of immediate hypersensitivity disease. Verapamil, flunarizine and diltiazem were all effective in inhibiting antigen-induced bronchospasm in the guinea pig; however, the effects seen were at relatively high doses compared to the doses known to cause cardiovascular effects. Nifedipine caused no significant inhibition of resistance or compliance changes induced by antigen. Flunarizine, verapamil and diltiazem were ineffective in inhibiting antigen-induced histamine release from rat peritoneal mast cells in vitro. Although these compounds were active inhibitors of 5-D-[5,6,8,9,H,12,14,15-3H(N)]-hydroxy-6,8,11,14-eicosatetraenoic acid production in rat basophilic leukemia-1 cells, only flunarizine and verapamil showed effects on the 5-lipoxygenase enzyme when assayed directly. Also, these compounds were ineffective on SRS-A mediated bronchospasm in vivo. These data suggest that the currently available calcium entry blockers have little potential use in immediate hypersensitivity reactions.