Michael, we had a huge discussion about this with PE many moons ago, and yes there is a mercapto which remains a trade secret. That much I was able to ferret out off that crafty wizard. I'd suggest a search but most times that doesn't find the relevant post. I think it was 'Microdol-X replacement".
To me, MDX has an ever so slightly smoother tone when used 1:0 and slightly more brilliance when used 1:3 as compared to Perceptol and D-23/25. Teeny tiny.