They are widely distributed throughout the plant kingdom, particularly in the Leguminosae and, to a lesser extent, in cereal grains and tubers. These substances inhibit the digestive enzymes trypsin and chymotrypsin. For example, raw soybeans contain a protein that inactivates trypsin and results in a characteristic enlargement of the pancreas and an increase in its secretory activity. It is this latter effect, mediated by trypsin inhibition that depresses growth. Clearly, soybeans and other related legumes should be properly cooked and processed before being eaten.
Potatoes-- which contain two major glycoalkaloid fractions, alpha-solanine and alpha-chaconine-- that have been exposed to sunlight show a significant increase in their alkaloid content. Solanine is a cholinesterase inhibitor and can cause neurologic and gastrointestinal symptoms, potentially including the fatal depression of the activity of the central nervous system.
For soybeans to serve as a good source of protein for feeding animals as well as humans, a certain amount of heat treatment or some other form of processing must be applied. This is because there are present in soybeans certain heat-labile factors that exert an adverse effect on the nutritional value of the protein.
The so-called protease inhibitors have received the most attention in this regard and have been shown to exert their anti nutritional effect in the short term by causing pancreatic hypertrophy and hyperplasia in the rat, the underlying cause for an inhibition of growth in these animals.
The prolonged feeding of raw soy flour or an enriched trypsin inhibitor fraction from soybeans to rats results in the development of hyper plastic and neo plastic nodules of the pancreas, including carcinomas. It should be emphasized that all of these adverse effects are seen when protease inhibitors are present in relatively high concentrations in the diet and may be completely unrelated to the anti carcinogenic effects seen at low concentrations of the Bowman-Birk inhibitor.