After my article on the role of carbon isotope testing in the Floyd Landis case, a question has come up several times in the comments and in my e-mail: since it's well-known that Landis was taking cortisone for his hip, could this have skewed the isotope ratios in his testosterone?
I doubt it very much, and here's why: first off, around 95% of the circulating testosterone in the male body is produced in the testes. For Landis's isotope ratios to be off a significant amount through something involving his own metabolic pathways, this is the only place that's worth looking.Testosterone and the other steroids are produced from cholesterol. The testes and other steroidogenic tissues have a stockpile of cholesteryl esters ready to be used for steroid synthesis, so it's going to be an uphill fight to alter things by any route, given that reserve.
Now it's time to dive into some biochemistry for the next few paragraphs - follow along if you like, or jump down to near the end if you don't want to see a lot of structures. OK, in steroid synthesis the first thing that happens is the chewing off of a side chain on the D ring to form pregnenolone, which is then turned into progesterone. That's the starting material for both testosterone and cortisol/cortisone. (Note that those last two are interconverted in the body by the 11-HSD enzymes).
Going down these different pathways, testosterone and cortisol end up with rather different structures. Cortisol's more complex. If you flip back and forth between those links in the previous paragraph, you'll see that the A and B rings are the same in both, but the C ring of cortisol has an extra hydroxyl group at C11, and it also has some oxidized side chain left at C17, which has been completely chopped off in testosterone. The question is, can you get from cortisol back to something that could be used to make testosterone?
I can believe the side-chain transformation much easier than the C-11 deoxygenation. Here's the metabolic fate of cortisol. Note that all these metabolites still have an oxidized C-11 - if anything is going to be recycled into testosterone, that C-11 is going to have to be reduced back down. And if there's a metabolic pathway that does that to any degree, I can't seem to find out anything about it. If it's a feasible pathway at all, it must be very minor indeed. If any steroid experts can shed light on this, I'd be glad to hear the details. (There's also the question of how long such intermediates would be available, versus their half-life before further metabolism and excretion, but that's a whole other issue).
No, if Landis's carbon isotope ratios are off significantly - and we haven't seen the official numbers yet - then it's hard for me to see how the cortisone injections could have much to do with it. We'll be stuck, in that case, with either conspiracy theories or with the conclusion that Landis used testosterone, and if it comes to that, I know which one I'm most likely to believe.