The Origins of Butadiene's Carcinogenicity

The chemical 1,3-butadiene is a workhorse in industry, particularly in the manufacture of synthetic rubber. But the Environmental Protection Agency designates it a carcinogen. The chemical threatens not only industrial workers, but also the general populace through sources such as cigarette smoke and automobile exhaust. Now researchers detail how a cellular pathway that usually detoxifies harmful chemicals actually creates a potent mutagen from butadiene (Chem. Res. Toxicol., DOI: 10.1021/tx100304f).

Despite intense research, the details of butadiene's mutagenic properties have remained murky for decades. Many studies have pointed to butadiene metabolites—particularly with one or both of the alkenes converted to epoxides.

Then 15 years ago, F. Peter Guengerich at Vanderbilt University in Nashville, Tenn., and colleagues reported an unexpected, mutagenic collusion between butadiene and a natural tripeptide called glutathione. In cells, this peptide reaches exceptionally high concentrations—5 to 10 mM—and one of its manifold functions is to help eliminate unwanted chemicals. Through glutathione's reduced cysteine residue, the enzyme glutathione S-transferase attaches the peptide to undesired molecules. The cell's housekeeping machinery uses this peptide handle to dispose of the offending chemical.

But Guengerich's team found that glutathione created problems with butadiene, rather than reducing them. When they incorporated mammalian glutathione S-transferase into Salmonella typhimurium, a standard bacterial species used in assessing mutagens, butadiene's mutagenicity increased. So they recently set out to uncover the molecular details.

Using liquid chromatography-mass spectroscopy, high-performance liquid chromatography, and nuclear magnetic resonance, the researchers isolated and structurally characterized the culprit: a glutathione conjugate with butadiene diepoxide (the metabolite containing two epoxides). They then compared this conjugate head-to-head with butadiene diepoxide in a S. typhimurium strain sensitive to base mispairing and found that the conjugate was hundreds of times more mutagenic than the metabolite alone.

"The prevailing dogma was that binding to glutathione reduced toxicity," Guengerich says. "But our work shows that it actually makes butadiene much more toxic."

Toxicologist Adnan Elfarra of the University of Wisconsin, Madison, says the study raises a new question for the field: "Why is this glutathione conjugate more mutagenic than the precursor, butadiene diepoxide, which is a cross-linking agent?"