Why do radishes burn your mouth?

There is an substance called aurevetic acid ,amongst others that metabolizes with oxygen when you bite into them ,it is the same stuff found in horseradish ,only much weaker and contrary to the belief of some they do NOT contain capsacin as this hemical is only produced by members of the pipperatta officianalis or pipperitta agusticeps genus of hot peppers and also in some forms of the pipperata capsacia.