Reply from Dr. Arpad Pusztai about Sialic Acid

From: Dr Arpad Pusztai
Sent: Thursday, March 29, 2001 6:02 PM
To: Lynne Schuler

Subject: Lectins

 

Dear Ms Lynne Schuler,

I am sorry for not replying to you earlier but I have been feeling unwell. Hopefully, it is not too late for you all.

As I understand there is an argument about the possible role of lectins in the diet on metabolism, etc.  I shall try to describe some of the principles of cell-lectin interactions.  Incidentally, you can get a great deal more from our book: Els J.M. Van Damme et al, 1998: "Handbook of Plant Lectins", John Wiley & Sons, Chichester, New York, etc.  ISBN 0-471-96445-X.

Lectins do react with cells including erythrocytes and other blood cells in a sugar-specific fashion which may or may not include blood group specificity.  However, there are only few lectins which recognize blood group determinant carbohydrate groups specifically.  Most lectins react with erythrocytes through non-blood group specific determinants.

Thus, it is very difficult to see how one can categorizes people of different blood group specificity into which food should be good or bad for them.  I am afraid, there are no shortcuts; one has to determine in vivo whether the lectin in the diet survives in the mammalian gi tract, whether it binds to the epithelium and whether this has any good or bad effect on the health of the individual. Some lectins are definitely toxic but some are definitely not.

For example, the snowdrop bulb lectin is not toxic for mammals because it specifically reacts with alpha-1,3 mannosyl residues and these are scarce in the mammalian gut epithelium.  However, I certainly would not like to forecast which lectin is toxic and which is not.  We have done a low resolution glycosyl map of the rat gi tract and published it but no similar map for humans exist.  From this one can draw some general guidelines but eventually all this must be verified by experimentation.

In the majority of instances sialic acid DOES NOT interfere with lectin binding.  A few lectins, such as peanut agglutinin, soybean agglutinin, etc. are inhibited when the terminal sialic acid masks a galactosyl non-reducing penultimate sugar.  In such cases the removal of the sialic acid by neuraminidase increases lectin reactivity.  But even with galactose-reactive lectins the fine specificity is due to bigger carbodydrate side chain structures and not just the terminal residue.  It would be nice to have a direct relationship established between food lectins and mammalian consumers and certainly blood groups would come into this picture but the situation would be far more complex. 

For the time being we must fall back on empirical observations of our grandmothers.  Unfortunately, most of us have already forgotten about what they used to tell us as little kids.  I once said that my grandmother would have laughed her head off if I told her that 20 years ago I got something like half a million dollars to find out whether uncooked kidney beans were good or not for us. I am afraid, that is the best I can say but the rest is quoted in the reference list and also in the book I mentioned.

Have a good fight!

Best regards to all

Arpad Pusztai