Q: I'd heard that metallic things like knives or blades will reduce the content of enzyme in fruits or vegetables. Is that true?-- Philip M.
I have not heard of any evidence to support this. In fact, the usual belief is quite the opposite, that cutting fruits or vegetables with a steel knife will cause them to turn brown. A common claim is that cutting leafy vegetables, like lettuce, will make them turn brown quicker than if you tear them by hand. Explanations are that the knife reacts with the food to turn it brown, or that cutting damages the cells of the leaf, or that it causes more bruising.
Proponents of ceramic knives say that their knives don't react with foods the same way that steel knives do.
Both of these claims might be true if you were using a pure carbon steel knife. Fortunately, most of the metal kitchen knives you will buy nowadays are made from stainless steel. The chromium and nickel added to carbon steel to make it stainless help keep the knife from reacting with foods, so the choice of cutting or tearing lettuce is more aesthetic than practical. Irregular chucks of raggedly torn lettuce may be more appealing to the eye.
For cut fruit, or vegetables like artichokes, there are a few to prevent enzymatic browning -- blanching the food, which will work for tougher vegetables but not on soft or delicate foods, or dipping the food in a solution of Vitamin C or citric acid and water (see Ascorbic Acid). You can also rub the fresh cut surface with the open face of a lemon which has been cut in half. A salt solution can be used, too, but the amount of salt needed will seriously affect the taste.
While it may be unusual to find non-stainless steel, it does happen. I have a Chinese cleaver that isn't stainless. It tends to rust more readily, but I don't know how it would work with fruits and vegetables, as I don't ever use it for that purpose.