Werner G. Jaffé
If one considers the varied biological activities of many proteins and peptides such as enzymes, hormones, transport proteins, and antigens, it is not surprising that some members of this large group of substances should exhibit toxic actions in organisms other than those in which they had been produced. Indeed, several proteins-bacterial toxins, snake venoms, and plant toxins (ricin)-are among the most toxic agents known.
The proteins are essential constituents of nearly all foods. Therefore, a discrimination among the nutrittonal effects caused by amino acid deficiency or imbalance, poor digestibility, and toxic effects may be difficulto Moreover, additional factors, such as protein inhibition of digestive enzymes, may play an important if still unknown rol e in determining whether or not a certain protein can exert a harmful action.
Most animals have evolved detoxification mechanisms against the action of foreign proteins. In a certain sense the immunological defense mechanisms can be regarded as such, a1though they may lead to the opposite effect, Le., hypersensitivity and allergic reactions. Another defense mechanism is gastrointestinal digestion. A1though it serves the purpose of primari1y transforming foods into components that can be absorbed through the intestinal wall, it simultaneous1y destroys the native structure of ingested proteins and peptides, abolishing their biochemical activities and possible toxicities.
Toxicity may be detected, therefore, in certain proteins or peptides existing in foods if they enter the body by other than the oral route or if they resist digestive destruction. They might possib1y contain toxic constituents not destroyed by digestion, like selenium, or the proteinaceous substances might act in some way on the food product before it is ingested, as in the destruction of vitamins ‘by certain enzymes.
The specific bio10gical activity of practically all proteins is destroyed by heat, although considerable differences exist between proteins with respect to their rate of heat inactivation. Consequently, most food products, if proper1y prepared, can be consumed without harmful effect. Notable exceptions are someof the allergy-producing foods, which will be dealt with later on, and the selenoproteins.
The mode of action of most of the toxic proteins is still obscure. In some cases specific in vitro activities can be detected, and considerable efforts have been made to relate these to in vivo effects. Some of the better known groups of proteins with recognized toxic or antinutritional action are bacterial toxins, animal toxins, hemagglutinins, enzyme inhibitors, vitamin-binding pro teins (avidin), vitamin-destroying enzymes; enzymes releasing toxic compounds, and selenium-containing proteins. Not all are discussed in this chapter. For further information on the other toxins the reader should refer to other chapters in this vo1ume.