Benzene is a heavily used industrial chemical, a petroleum byproduct, an additive in unleaded gas, and a ubiquitous environmental pollutant. Benzene is also a genotoxin, hematotoxin, and carcinogen. Chronic exposure causes aplastic anemia in humans and animals and is associated with increased incidence of leukemia in humans and lymphomas and certain solid tumors in rodents. Bioactivation of benzene is required for toxicity. In the liver, the major site of benzene metabolism, benzene is converted by a cytochrome P-450-mediated pathway to phenol, the major metabolite, and the secondary metabolites, hydroquinone and catechol. The target organ of benzene toxicity, the hematopoietically active bone marrow, metabolizes benzene to a very limited extent. Phenol is metabolized in the marrow cells by a peroxidase-mediated pathway to hydroquinone and catechol, and ultimately to quinones, the putative toxic metabolites. Benzene and its metabolites appear to be nonmutagenic, but they cause myeloclastogenic effects such as micronuclei, chromosome aberrations, and sister chromatid exchange. It is unknown whether these genomic changes, or the ability of the quinone metabolites to form adducts with DNA, are involved in benzene carcinogenicity. Benzene, through its active metabolites, appears to exert its hematological effects on the bone marrow stromal microenvironment by preventing stromal cells from supporting hemopoiesis of the various progenitor cells. Recent advances in our understanding of the mechanisms by which benzene exerts its genotoxic, hematotoxic, and carcinogenic effects are detailed in this review.

Although benzene is best known as a compound that causes bone marrow depression leading to aplastic anemia in animals and humans, it also induces acute myelogenous leukemia in humans. The epidemiological evidence for leukemogenesis in humans is contrasted with the results of animal bioassays. This review focuses on several of the problems that face those investigators attempting to unravel the mechanism of benzene-induced leukemogenesis. Benzene metabolism is reviewed with the aim of suggesting metabolites that may play a role in the etiology of the disease. The data relating to the formation of DNA adducts and their potential significance are analyzed. The clastogenic activity of benzene is discussed both in terms of biomarkers of exposure and as a potential indication of leukemogenesis. In addition to chromosome aberrations, sister chromatid exchange, and micronucleus formation, the significance of chromosomal translocations is discussed. The mutagenic activity of benzene metabolites is reviewed and benzene is placed in perspective as a leukemogen with other carcinogens and the lack of leukemogenic activity by compounds of related structure is noted. Finally, a pathway from exposure to benzene to eventual leukemia is discussed in terms of biochemical mechanisms, the role of cytokines and related factors, latency, and expression of leukemia.

A curious feature of insect blood, shown by many analyses, is the apparent absence of all but minute amounts of sugar (for reviews, see Beutler, 1939;Babers, 1941;Buck, 1953). Reducing substances are abundant, but (except in the case of a few species) the greater part of these are not fermentable by yeast and are therefore presumably not sugar. Typical are data on the silkworm, Bombyx mori, in which the reported levels of blood sugar range from zero up to about 30 rag. per 100 ml. (Florkin, 1937;Kuwana, 1937; and others) --remarkably small amounts for a metabolically active animal with a high dietary intake of sugars. Twenty years ago, however, Kuwana (1937) made the significant discovery that acid hydrolysis of Bombyx hemolymph caused release of much reducing sugar from a substance which was not glycogen. Even earlier, Ronzoni and Bishop (1929) had reported an unidentified non-reducing "polysaccharide below glycogen" in the blood of honeybee larvae. These reports received little attention. Recently, unaware of them, we made essentially the same finding. The blood of Bombyx mori and of two other insect species was found to contain large amounts of anthrone-reactive material which was neither a reducing sugar, sucrose, nor glycogen (Wyatt, Loughheed, and Wyatt, 1956).As already reported in preliminary form (Wyatt and Kalf, 1956), we have now identified as a major sugar of insect plasma the non-reducing disaccharide a,a-trehalose. In the present paper, we give details of the isolation and characterization of this sugar together with some quantitative analyses of trehalose and certain other carbohydrates in insect plasma. Their distribution and significance will be considered.

Benzene expresses its carcinogenic potential in humans largely in the form of acute leukemia. Because an understanding of the formation of DNA adducts by benzene metabolites may help to explain the etiological role they play in benzene-induced bone marrow disease, we have synthesized, isolated and characterized adducts formed by the reaction of deoxyguanosine with hydroquinone and p-benzoquinone, two toxic metabolites of benzene. [3H]Deoxyguanosine and [14C]hydroquinone reacted in neutral aqueous buffer containing iron to form two dual-labeled products, which were separated using HPLC. When p-benzoquinone was substituted for hydroquinone, the same adducts were formed in the absence of added iron. The ultraviolet and fluorescence spectra of the less polar adduct, called Adduct 2, were distinctly different from the spectra of the starting materials. NMR and mass spectrometry suggested a compound with a mass of 357 with the p-benzoquinone moiety bound to the N-1 and N2 positions of deoxyguanosine. Based on these data it is proposed that Adduct 2 is (3'OH)benzetheno(1,N2)deoxyguanosine. The more polar product, Adduct 1, was found to have a unique ultraviolet spectrum but did not appear to be fluorescent. Both adducts were observed after calf thymus DNA was incubated with hydroquinone and digested to its constituent nucleosides.

Rat liver mitoplasts were incubated first with [3H]dGTP, to form DNA labeled in G, and then with [14C]benzene. The DNA was isolated and upon isopycnic density gradient centrifugation in CsCl yielded a single fraction of DNA labeled with both [3H] and [14C]. These data are consistent with the covalent binding of one or more metabolites of benzene to DNA. The DNA was enzymatically hydrolyzed to deoxynucleosides and chromatographed to reveal at least seven deoxyguanosine adducts. Further studies with labeled deoxyadenine revealed one adduct on deoxyadenine. [3H]Deoxyguanosine was reacted with [14C]hydroquinone or benzoquinone. The product was characterized using uv, fluorescence, mass and NMR spectroscopy. A proposed structure is described.