Why does heating kidney beans at a low temperature potentiate the toxicity of phytohaemagglutinin?

Question

I recently found out that red kidney beans contain a relatively high amount of the toxic lectin, phytohaemagglutinin.

An article on phytohaemagglutinin on the FDA website states:

Several outbreaks have been associated with "slow cookers" or crock pots, or in casseroles which had not reached a high enough internal temperature to destroy the glycoprotein lectin.

It has been shown that heating to 80°C may potentiate the toxicity five-fold, so that these beans are more toxic than if eaten raw.

What I do not understand is how the toxicity could increase if the beans are cooked at lower temperatures in a crock pot. I would understand if the toxicity remained the same (that is, not be deactivated), but I thought a toxin is nonliving (i.e., unable to grow and reproduce) since it is not a bacteria?

I know nothing about biology, so I am utterly confused.

Answer 1

I am a Snr lecturer and a PhD candidate. Currently, I am putting my theses together. My research area involve in part the kinetic degradation of lectins in neglected and underutilized legumes. I also looked at the possibility of using irradiation to produce complexed degraded starches to inactivate lectins during low temperature extruder cooking.I observed in some of the extruded samples that lectins have actually potentiated up to about 50%. I was confused. I know lectins are proteins so they are supposed to denature when heated. I was looking around to see whether someone has made similar observation and I saw that indeed somebody has reported similar observation. I was so over joyed but it was short lived-no explanations were offered. I now offer a possible explanation. Yes, lectins are proteins(or glycoproteins) and like most proteins their tertiary and quaternary structures are 3D. So the 3D forms have a couple of smaller subunits, woven together into the giant super structure(lectin). In the 3D form, most of the binding sites are "buried" or not fully exposed. It is possible that these smaller units have binding sites for carbs and are best positioned to present their ligands for exposure and maximal binding when the 3D structure begins to "melt down". This melt down occurs at low temperature when denaturation has actually started. Thus, the greater the exposure number of subunits bearing the binding sites, the greater the potentiation. My work used low temperature extrusion cooking and the observation occurred in the extrusion cooking.

I just hope this suggestion is adequate. That is indeed the explanation I am going to offer for my observation anyway. Wish me best of luck!

Bye!