- Naturally Occurring Toxic Compounds in Foods
A. Plant foods
(1) Protease inhibitor: trypsin and chymotrypsin inhibitors
what do they do
where are they found?
what is their function in plants
- trypsin and chymotrypsin are digestive enzymes secreted by the small intestine and their function is to partially hydrolyze proteins/polypeptides (energy, essential biological components)
anti-nutritional factor (reduce dietary protein digestibility energy reduction, reduced bioavailability of essential amino acids, toxicity)
- these proteins are found in the seeds of grain legumes (pulses and oilseed legumes): beans (Phaseolus spp.), lentils, chickpea, soybeans - also present in corn,peanuts, barley , squash and potatoes
- legumes/pulses are an extremely important source of dietary protein in many parts of the world (Africa, Bangladesh, India, Middle East)
protein concentration ranging from 20-40%
-> meat (14-40%); milk (3%) wheat (11%)
Functions in plants
- storage protein - defense mechanism against insects and herbivores
) Protease inhibitors: trypsin and chymotrypsin inhibitors
Inactivation/Destruction
Toxicity/limitations
\
Benefits
- moist heat treatments inactivates these proteins (100 c/3-10 minutes)
- dry heat is much less effective (still active after 18 hours of dry heat)
- plant breeding and genetic engineering to reduce the levels of proteases inhibitors
reduced nitrogen and sulfur absorption
- reduced bioavailability of essential amino acids - reduced growth rate in lab animals - protein malnutrition - induces pancreatic hypertrophy (increase in organ size due to cellular proliferation [hyperplasia]) possible relationship to pancreatic cancer
- genetically engineered into plants (e.g. potato) as biological control agent (combat insect and pathogen damage)
- correlative evidence for their anti-carcinogenic activity ([a]block the formation of reactive oxygen species; [b]deprive rapidly growing cancer cells of protein/amino acids) breast cancer treatment (spurred the consumption of pumpkin and squash seeds)
- provide additional nutritional value when denatured
correlative evidence for their positive role in AIDS treatment virus becomes active through an aspartic acid protease (inhibition via pi? importance in developing countries?)
- Naturally Occurring Toxic Compounds in Foods
(2) Hemagglutinins
what are they
found where?
Function(s) in plants
\Inactivation/Destruction
- also known as lectins and phytohemagglutenins
- plant glycoproteins (CHO: 4-14% of molecular mass)
- found in a wide variety of seeds, vegetables and fruits (e.g. kidney, castor, navy and soybeans, peas, lentils, peanuts, barley, wheat, rice and potato); also found in viruses, bacteria and molds;
->found on the surface of influenza viruses -> bind to mammalian
cells (fusion protein) -> molecular grapping hook)
Function(s) in plants
- basically unknown - possible roles in: insect resistance; and carbohydrate storage and translocation (during growth)
Inactivation/Destruction
- moist heat treatment
(2) Hemagglutinins
Toxicity
resistant (40-90%) to stomach acid and proteolytic activity and pass to the small intestine intact
- bind to the carbohydrates present on red blood cells(a, b, ab, o)-> agglutination (a) anemia; kidney(blocks kidney filtration) failure/death; and (b) bind to intestinal epithelial cells (villi) interfere with nutrient (e.g. protein) absorption [chronic consumption destruction of villi, resulting in abnormal growth and development (severe nutrient deprivation)]
bind to cells that line the large intestine lesions that permit the intestinal bacteria to enter the blood stream -> liver and other organ damage (example e.coli)
- LD50 (mice and rats): 3-10 g/kg bwt - "in general, all dietary lectins should be treated as potentially harmful and should where possible be denatured before they are consumed" (FAO/WHO; 2011)
ricin (castor beans) lethal dose (mice): 0.001 mcg/kg of bwt
ricin and the umbrella murder (1978)
- 2013 (USA): letters containing ricin sent to President Obama, and select Senators and members of Congress, and the mayor of New York
(3) Amylase Inhibitors
what are they
what are they found in
proteins (glycoproteins) that inhibit the activity of mammalian -amylase in the small intestine reduces starch absorption
- this mammalian enzyme hydrolyzes starch (a-14 glycosidic linkages) to produce glucose and maltose (readily absorbed)
- Present in/isolated from [dietary supplements] phaseolus vulgaris (e.g. navy and white kidney beans)
used as a dietary supplement weight loss, glycemic index lowering [sugar blocker diet]
-possible diabetic treatment
Amylase Inhibitors
Function(s) in plants
Inactivation/Destruction (how to get ride of them)
Toxicity
Benefits
Function(s) in plants
- insect resistance-> impede starch digestion (energy source)
Inactivation/Destruction (how to get ride of them)
- moist heat treatment - reduced bioavailability of an important energy source for many humans - > 20%reduction in animal weight gain observed with 100iu/100g in feed (levels of up to 100 U in 100g of beans) - organs (liver and pancreas) weight changes (impact on function?) - genetically engineered into plants as biological control agents (combat insect damage)- > GMO
