I have found some interesting discussion about marinating. For the most part, apparently, it usually doesn't do much to tenderize and sometimes it actually dries it out.
If you have found, through your own experience, that this is not true, please post it. I would love to try your ideas.
I guess that this is the reason that I have never found marinating meat ever really tenderized it. I thought I was doing it wrong.
The discussions that I have read do not, however, discredit the value of adding flavor. The process can carry flavors into the meat but they do not permeate very far. The chemical process such as the use of vinegar, orange juice, wines, lemon juice and the like is commonly referred to as using "flesh eating enzymes". Although a bit unnerving in it's nomenclature, it is the process of using a chemical to break down connective tissue in meat to loosen the fibers. This allows the mixture to flavor the meat.
As Harold McGee, author of On Food & Cooking (Canada, UK), says, "In general, there are at present no really satisfactory ways of tenderizing meat chemically."
On Food and Cooking: The Science and Lore of the Kitchen