(VCCEP) Tier 1 Pilot Submission for BENZENE - Tera

derived metabolites in target organs. Benzene is known to be metabolized in the lung (Snyder
and Hedli, 1996 Sheets and Carlson, 2004; Powley and Carlson, 1999, 2000, 2001) and rabbit
bone marrow (Andrews et al., 1977). It is probable that benzene itself is not metabolized in
human marrow, which lacks the P450 enzyme CYP2E1 (Genter and Recio, 1994). A current
map of the pathways for benzene metabolism is shown in Fig. 1 (from Ross, 2000). This map is
simplified to focus on metabolites that are thought to be important for toxicity and does not
include the conjugation pathways and their products that are excreted or, perhaps, are further
metabolized in target organs. Metabolites that have been proposed as being responsible for
benzene toxicity include: 1) benzene oxide (epoxide), 2) mucondialdehyde, 3) quinones (e.g.,
benzoquinone, p-benzoquinone 4,4’-diphenoquinone), and 4) polyphenols (e.g., benzene
dihydrodiol, catechol, trihydroxybenzene).
Benzene oxide is a highly reactive intermediate and would be expected to form adducts with
proteins in the vicinity of its synthesis and with DNA, and it is difficult to explain why toxicity
would be higher in bone marrow than in liver. Also, stable DNA adducts in rodents are not
detected consistently by the 32 P postlabeling technique after acute benzene exposure or after
long-term administration (Reddy et al., 1990, 1994), although protein adducts are observed
(McDonald et al., 1994) even in untreated animals.
Small amounts of mucondialdehyde are probably produced in vivo from benzene (Kline et al.,
1993), and it is a reactive intermediate, but there is no evidence that it is involved in benzeneinduced
toxicity or carcinogenicity.
Polyphenols are produced in large amounts via benzene metabolism and are released into the
blood either as the phenols themselves (phenol, benzene dihydrodiol, catechol
trihydroxybenzene, and catechol) or in conjugated form (e.g., as sulfate or glucuronide) where
they can subsequently reach target organs (Ross, 2000). Reactive phenolic benzene
metabolites such as hydroquinone and trihydroxybenzene produce DNA damage in human
myeloid cells in vitro similar to that seen in mouse bone marrow by benzene (Kolachana et al.,
1993). Phenol itself is probably not solely responsible for the toxic effects of benzene, because
the administration of phenol alone does not reproduce the myelotoxicity caused by benzene
(Tunek et al., 1981). However, concomitant administration of both phenol and hydroquinone
does reproduce the toxicity (Eastmond et al., 1987; EPA-NCEA, 1998). It is plausible that the
target-organ and target-cell toxic effects induced by benzene in vivo result from polyphenols,
based on known effects of the polyphenols and the known enzymology (Ross et al., 2000).
p-Benzoquinone might be a key reactive intermediate in the formation of acute myeloid
leukemia, because only the myeloid cell line has the enzymatic capacity (e.g., myeloperoxidase)
to convert hydroquinone to p-benzoquinone (Ross et al., 1996). Also, p-benzoquinone is a
known clastogen (Smith et al., 1989). Hydroquinone can enhance the number of myeloid
progenitor cells, which could then increase the conversion of hydroquinone to p-benzoquinone,
thereby raising the cellular concentration (Irons and Stillman, 1996). Because the formation of
stable DNA adducts has not been detected in vivo for the highly reactive p-benzoquinone, a
potential mechanism for leukemogenic effects might be through protein binding, such as to
histone protein, which in turn, might produce chromosomal damage. This type of mechanism
might be considered a secondary toxic effect for which a threshold could exist.
The main conclusions regarding metabolism from the EPA IRIS benzene assessment are:
1. It is generally agreed that chronic toxicity of benzene in animals and humans results
from the formation of reactive metabolites.
Benzene VCCEP Submission
March 2006
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