I caught a glimpse of an Alton Brown show where he mentioned that enzymes were more important than anything else in breaking down tough meats.
I seem to recall him mentioning that Papaya and Pineapple as being Enzyme-rich and good for marinades.
I buy a lot of tough meats for the BBQ to make steak tacos, etc. and would like to verify his info. or seek other thoughts on the subject.