2006

Isotopic Distribution

The kinetic isotope effect, or the change in a chemical reaction’s rate when an atom in one of the reactants is substituted with one of its isotopes, has popped up in some unexpected circumstances (and for some people, rather disconcerting ones). The difference between the carbon-12 and carbon-13 isotopes, for example, isn’t large, but because a cell’s carbon compounds are constantly being processed, the ratio between the two in the various carbon atoms of a biomolecule (the isotopic distribution) is noticeably different in samples from living creatures: the lighter carbon-12 has been getting enriched for a billion years as plants grow on the remains of other plants, as animals eat them, and so on. In fact, mass spectrometry techniques can even tell whether a given molecule was produced from the biochemistry of tropical plants or from that of temperate-zone ones.

This aspect of analytical chemistry made headlines in 2006 when American cyclist Floyd Landis, original winner of that year’s Tour de France, was accused of having taken testosterone supplements to enhance his performance. Landis claimed that his body naturally tended to produce more testosterone and that he had not boosted his biochemistry. But mass spectrometry told another story.

A testosterone molecule has nineteen carbon atoms. A bit over 1 percent of all carbon atoms are carbon-13. So in the absence of isotope effects, statistics would say how many testosterone molecules in a sample would contain two carbon-13s (about one in ten thousand), how many would have three (about one in a million), and so on. But steroid molecules from natural sources have more carbon-12 than you would predict, and the pattern of this enrichment depends on the biochemical pathways that formed them.

Landis’s testosterone did not show the human pattern, but one expected from warm-climate plants. And since synthetic testosterone is produced from warm-climate plant sterols (yams and soy), it has a completely different profile from the human variety. In 2010, Landis finally admitted to taking testosterone and other performance-enhancing drugs (but maintained that the 2006 results were the result of lab errors).

Floyd Landis racing in a time trial in California, 2009. A long list of professional bicycle racers have had their careers tarnished by evidence of performance-enhancing drug use, thanks to modern analytical chemistry.