When vegetables are frozen they are blanched to stop/greatly retard the enzymatic actions before they are frozen. I believe this is also true for some/most fruits ...
Canned foods are also safe, regarding the loss of nutrients due to enzymes, since they are either pre-cooked before canning or are actually cooked in the can during canning - but the processing leaches vitamins and nutrients into the liquid they are packed in ... so unless you incorporate the can liquor into the dish in some way - you're probably throwing away more nutrients than the enzymes consumed.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain