Resistance of plant lectins to proteolytic degradation
Portions excerpted from: Freed DL. Dietary Lectins and Disease, in: Food Allergy and Intolerance, Brostoff and Challacombe Editors, 1987
Dietary proteins are rapidly degraded during passage through the gut by digestive enzymes in the small bowel. Any residual undigested matter is then degraded by bacteria in the large intestine and used as sources of energy. In contrast to dietary proteins, lectins resist degradation in the small intestine () and are also resistant to breakdown by most gut bacteria. Thus, as most lectins survive at least in part the passage through the digestive tract in an immunologically and functionally intact form they can exert their potent biological activities in vivo. Similar to the enhanced stability of hormones and growth factors bound to receptors, lectins are also protected by receptor-binding. However, this is not necessary as shown by the almost complete survival of the agglutinin from Galanthus nivalis, snowdrop bulbs (GNA) which does not bind to the gut wall on acute oral exposure suggesting that resistance may also result from particular features of the molecular structure ().
As part of the normal turnover of the gut epithelium, cells are shed from the villus tips into the lumen and most cellular material is then digested and recycled. The presence of lectins attached to these cells does not interfere with the breakdown of cell contents but the liberated lectin can move further down the gut and bind to the next receptor with an appropriate carbohydrate moiety. Although lectin-binding is most frequently studied in the small intestine, similar binding can occur throughout the entire digestive tract, from the stomach to the distal colon. However, as surface glycosylation varies in the different functional parts of the gut, lectin-binding is not uniform in the digestive tract. Thus, when a particular glycosyl group is absent from the surface of the small bowel but present on the large intestinal epithelium, specific lectin-binding will almost exclusively be to the wall of the colon and vice versa. Accordingly, with the appropriate choice of lectins, it is possible to selectively affect the metabolism of different parts of the gut.
As some lectins are partially heat-stable and most remain active during the passage through the gut, interactions between lectins in the diet and the gut can occur, occasionally with dramatic consequences. Although it is known from animal studies that lectins can damage the gut and this may lead to various nutritional disorders (2), it is not generally appreciated that this only occurs at relatively high lectin concentrations in the diet. However, lectins can also have beneficial effects particularly at low dietary inclusions, and these may find uses in medical/clinical practice ().
Resistance to cooking
Many lectin-containing foods evade cooking because they are normally eaten raw, such as the tomato (Lycopersicon esculentum), which is now the primary vegetable source of vitamins and minerals in the USA. After mastication of one tomato, its lectin is bound to all oral mucous membranes. This N-acetylglucosamine-specific lectin is resistant to pepsin and trypsin and tolerates a pH range of 1.5-9.0. () It inhibits the mitogenic effect of other lectins on animal lymphocytes, and in their absence suppresses lymphocyte metabolism.
Consumption of raw foods
Raw and relatively unprocessed foods are becoming more fashionable among health-conscious individuals in the Western world. Given the abundance of food in prosperous countries, such persons are therefore exposed to greater doses of dietary lectins than at any time, probably, in human evolution. The current fashion for sprouting beans and grains is therefore to be encouraged in this context, since sprouting in most cases causes a sharp diminution of lectin content within a few days (). Nevertheless, occasional outbreaks of food poisoning due to the lectins of uncooked or part-cooked beans are reported (,). The average American consumes around 200 mg of lectin per year from tomatoes alone (), and many other salad ingredients are rich in lectins.
Although many lectins are destroyed by normal cooking (which is why grains and beans are edible), many are not. Relative resistance to heat was part of the classic description of wheat germ agglutinin (WGA) made by Joseph Charles Aub and colleagues in 1963 (), and enabled them to distinguish it from the wheat germ lipase from which they got it. WGA in fact is one of the more heat-sensitive wheat lectins, being destroyed after 15 minutes at 75 degress C whereas the lectins recently reported by Concon et al () in wheat glutenin and gliadin resist autoclaving at IMC for 30 minutes, and Freed has observed agglutinins in extracts of fresh baked bread (unpublished observations). Purists might object that not all that agglutinates is a lectin because certain plant and other lipids can cause a clumping phenomenon, indistinguishable from true haernagglutination, by partly dissolving portions of the phospholipid bilayer of the red cell membrane () However, although Concon et al () did not ascertain a sugar specificity for their autoclave-resistant agglutinins, they had previously removed lipids from the flour by extraction in butanol.
Three groups of workers (12,, ) have made exhaustive searches of food plants for lectins, identifying over 100 at the last count. Of these, Gibbons and Dankers, (11) noted that seven of them were autoclave-resistant (wheat bran, carrot, apple, canned maize, wheat flour, pumpkin seeds and banana). The banana agglutinin was actually enhanced by heating, and was inhibitable by N-acetylglucosamine (GNAc) and N-acetylgalactosamine (GalNAc). Nachbar and Oppenheim (4) also noted haemagglutinins in dry roasted peanuts, as well as in Com Flakes, Rice Krispies and Kellogg's Special K (which are all heated during manufacture). Avocado (Persea americana) lectin also resists the autoclave (14).
Phytohaemagglutinin (PHA) in kidney beans (haricot or navy beans) resists mild cooking in the whole beans, surviving up to 4 hours at 70'C with no loss of activity and retaining some activity even at 90 C after 3 hours. Beans that had been presoaked overnight before cooking lost all lectin activity after 10 minutes at 100'C, but if they were boiled without this pre-soaking some activity remained after 45 minutes. Young rats reared on a diet that contained part-cooked beans (80'C, 3 hours) went into negative nitrogen balance and lost weight instead of growing. 'Slow cookers', which can cook beans to perfect culinary standards, operate at 60-85'C, which is well within the danger range for PHA. There is however no detectable lectin in textured soya protein (Freed, unpublished observations) or in commercial soya-based infant formula feeds, because of the very high temperatures used in the processing.
Several of the above workers have noted year-to-year and batch-to-batch variation in lectin content of various foods, so the occasional lectin accident is likely to occur even with foods normally considered to be safe.
Relationship between survival and binding of plant lectins during small intestinal passage and their effectiveness as growth factors
Digestion 1990;46 Suppl 2:308-316 Pusztai A, Ewen SW, Grant G, Peumans WJ, van Damme EJ, Rubio L, Bardocz S Rowett Research Institute, Bucksburn, Aberdeen, Scotland, UK.
- The effects on the small intestine and the growth of rats of six pure plant lectins: PHA (Phaseolus vulgaris); SBL (Glycine maxima); SNA-I and SNA-II (Sambucus nigra); GNA (Galanthus nivalis) and VFL (Vicia faba), covering most sugar specificities found in nature, were studied in vivo. Variable amounts, 25% (VFL) to 100% (PHA, GNA) of the lectins administered intragastrically, remained in immunochemically intact form in the small intestine after 1 h. All lectins, except GNA, showed binding to the brush border on first exposure, although this was slight with VFL. Thus, binding to the gut wall was not obligatory for resistance to proteolysis. Exposure of rats to lectins, except VFL, for 10 days, retarded their growth but induced hyperplastic growth of their small intestine. The two activities were directly related. PHA and SNA-II, whose intestinal binding and endocytosis was appreciable after 10 days of feeding the rats with diets containing these lectins and similar to that found on acute (1 h) exposure, were powerful growth factors for the small intestine.