Probing The Molecular Origins Of Mutagenicity

A pollutant in diesel exhaust, the potent carcinogen 3-nitrobenzanthrone (3-NBA), has a closely related structural cousin that often appears at higher concentrations in urban air. A new Chemical Research in Toxicology paper (DOI: 10.1021/tx100052d) explains why the structural similarity is only superficial: the isomer 2-nitrobenzanthrone (2-NBA) poses much lower risk because key enzymes in the human body do not readily metabolize it.

A variety of combustion processes create 3-NBA, which is part of the molecular mix on fine soot particles in diesel fumes. By contrast, 2-NBA is largely the product of a nitration reaction that happens spontaneously in the atmosphere. The parent molecule, benzanthrone, is common in urban air, so 2-NBA often approaches concentrations ten times higher than those of 3-NBA.

Because of 3-NBA's mutagenicity, high ambient concentrations of a structurally similar molecule worry scientists. But 2-NBA simply has not exhibited the same potency. In an assay involving a Salmonella bacterium, for example, Marie Stiborová, of Charles University in Prague, found that 3-NBA was some 2000 times more mutagenic than 2-NBA.

Stiborová's team sought to understand why. The researchers expected the isomers' differences in physical properties to be minimal given their structural similarity. So they instead focused on the interaction of 2- and 3-NBA with enzymes.

The team incubated each isomer with DNA and a variety of enzymes, and then used thin layer chromatography to determine the extent to which the DNA had been altered. Because 2-NBA did not form potentially harmful DNA adducts, the researchers determined that it is not a substrate for enzymes that activate 3-NBA toward DNA. In experiments with intact human liver cells, 3-NBA again generated DNA adducts, whereas 2-NBA did not.

The researchers then modeled the interaction between each isomer and the active site in the crystal structure of NAD(P)H:quinone oxidoreductase 1, which is the most productive enzyme in activating 3-NBA to create DNA adducts. This molecular docking study shows that a key hydride transfer from the enzyme to the pollutant's nitro group would have to occur over a larger distance with 2-NBA than with 3-NBA.

Takeji Takamura-Enya, of the Kanagawa Institute of Technology, in Japan, who first identified 3-NBA's powerful mutagenicity, welcomes the contribution. "Unfortunately, research on this new group of mutagens—the nitro-aromatic ketones—had previously not been going well because of analytical difficulties," he says. "It's essential that we elucidate the hazards posed to humans by the toxic compound 3-NBA and its structural relatives as quickly and as thoroughly as possible."