Plant inhibitors of digestive enzymes

GALLAHER D, scHNEEMAN B o (1986) Nutritional and metabolic response to plant inhibitors of digestive enzymes. Adv Exp Med Biol. 199 167-84. 

NUTRITIONAL AND METABOLIC RESPONSE TO PLANT INHIBITORS OF DIGESTIVE ENZYMES... 

Gallaher, D., and Schneeman, B. 0. (this volume). Nutritional and metabolic response to plant Inhibitors of digestive enzymes. 

Some of the best investigated anti-nutrients are the enzyme inhibitors present in legumes and other plants. The Bowman-Birk and the Kunitz inhibitors of trypsin and other proteases are among the best characterized. In contrast to the non-specific and widespread influences of tannins and lectins (Carmona, 1996), the Bowman-Birk, Kunitz and other such inhibitors target specific enzymes. Corresponding with this, proteases and other digestive enzymes vary in sensitivity to the different inhibitors. 

The nutritional value of a protein also depends on its digestibility here again, plant proteins tend to be inferior to animal proteins. Their poor digestibility results from (1) the presence of peptide bonds which are relatively resistant to the digestive enzymes, (2) the presence of enzyme inhibitors such as the trypsin inhibitor present in soya beans (page 177), and (3) the presence of fibre which hinders the access of proteolytic enzymes (page 130). 

Plant protein sources provide 65% of the world s supply of protein, with cereal grains (47%) and pulses, nuts and oilseeds (8%) as the other major sources. Of the cereals, wheat (43%), rice (39%) and maize (12%) are the main contributors. Other limited sources of plant protein are fruits, leaves, tubers and other parts of plants included under the terms fruits, vegetables or root crops. Plant protein sources can differ from animal protein sources in terms of digestibility, amino acid composition, the presence of antinutritional (such as enzyme inhibitors) and toxic factors (e.g. saponins, cyanogens and lectins), which adversely influence protein digestibility, nutritional value and food safety. 

Protease inhibitors (Pis) represent a second class of proteins that may be expressed at insecticidal levels in GM plants [e.g. 37, 50-52], although none has been commercialized for this apphcation as yet [53]. Pis can be isolated and purified from many different plants, animals, and microbes. When ingested by insects, some Pis can inhibit their digestive proteolytic enzymes, causing starvation and death [e.g. 54-60]. 

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