This is a nifty article posted by Newport some time ago. There's an illustration on page 3 that shows where selenomethionine has its effects, then where SeMC has its effects. If you have a lot of selenomethionine left over after its other reactions, it might affect the level of SeMC. But SeMC, to the best of my knowledge, cannot go backwards and function as selenomethionine. For extra cancer protection, I can see the use for supplementing SeMC, but I wouldn't do without selenomethionine for my everyday life.