wrote:
Maybe this belongs in rec.bicycles.racing.chemistry.organic.synthesis.

From what I've been reading, the isotope ratio of synthetic
testosterone is only somewhat different than that of natural
testosterone. Does anyone know how much the difference is? And better
yet, why is there a difference? I am assuming that all carbon in the
biosphere has the same C12/C13 ratio, and that the difference in masses
is unlikely to produce any kinetic differences during the biosynthetic
reactions, in stark contrast to H1/H2 isotopes.

Is it that a starting material is taken from the soy source and then
modified with petroleum derived reagents (which have no C13 since they
have been in the ground for millenia)?

The difference is small (~3 parts per thousand PDB), but readily
measurable if you've got good technique. It arise from the fact that
synthetic testosterone is produced from plant sterols, which are lower
in 13C than animal hormones/tissues/etc. due to isotopic
discrimination.

Andy Coggan