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In school we learned that all ingested proteins are digested via denaturation and breaking up the larger macro-structure into peptides and amino-acids so small that they can be absorbed and which are then re-used by the body for what ever it needs these basic building blocks. This is supposedly the fate of all larger protein blocks and necessary as only small peptides and amino-acids can enter the blood stream.

While this may be the normal case for normal protein digestion, this model may turn out to be too generalised. For example, if we look at certain prion-diseases it seems possible that the causative agent for these diseases are infectious proteins.

If these proteins that cause kuru, Creuzfeldt-Jakob, BSE can be spread via ingestion of infected material then they seem to 'survive' the digestion process.

Scrapie modes of transmission have been debated for many years. Although experimental transmission can take several forms, the natural transmission of scrapie horizontally between individuals occurs through direct contact between animals and through contact with environmental contamination (reviewed in Schneider et al., 2008). Scrapie is predominantly acquired through the oral route, and the placenta and amniotic fluid are the most common sources of oral infection, although fetal parts, feces, and milk have all shown infectivity (see Schneider et al., 2008).
From: K.S. MacLea: "What Makes a Prion: Infectious Proteins From Animals to Yeast", International Review of Cell and Molecular Biology, Volume 329, 2017, p227-276.

That large proteins are active via the oral route seems to contradict the basic principles of protein digestion. Questions that arise here: Do some proteins survive human digestion? Which proteins survive? How or why do they survive?

To boil that down: is the general model of protein digestion and absorption incomplete with regard to larger protein structures? Wikipedia simply states that

PrP found in infectious material has a different structure and is resistant to proteases, the enzymes in the body that can normally break down proteins.

  • Sounds like a question suited for biology.se – Graham Chiu Jun 4 '18 at 3:04
  • From the question I can understand your predicament. Protein is required to build muscle mass and also to gain weight, it acts as a building block of our body and hence it is required to be consumed on a regular basis. For digesting protein our body's digestion system needs to work overtime. This makes large amounts of protein to be digested very difficult and can be bad for our entire digestion system in the long run. – GiGuy Oct 26 '18 at 9:47
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    @GiGuy - I have converted your answer to a comment. If you would like to repost it as an answer, we expect assertions to be backed up with science and references, such as your claim that "digesting large amounts of protein is difficult" and detailing how it is bad for the system. – JohnP Oct 26 '18 at 14:54
  • "To boil that down: is the general model of protein digestion and absorption incomplete with regard to larger protein structures?" -- Well, it would seem that it is from the references you cited, but have you missed something? It seems unlikely to me that research in this field hasn't asked this question. – Carey Gregory Oct 27 '18 at 0:45
  • @CareyGregory The background leading to this question is the dispute over whether ingested enzymes can work at all vs CJD v school wisdom: "every protein is broken down to amino-acids" (that was actually my profs dismisssive command, shaking head amused, when I asked him about enzymes surviving digestion, entering blood stream – despite obvious infections, side-effects of ingested enzymes etc.) For years I have tied to (re)search this, no convincing evidence either way and esp not addressing this conundrum. I may be forest blind for trees, of course by now. – LаngLаngС Oct 27 '18 at 0:56
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Do some proteins survive human digestion?

Yes. Prions are misfolded proteins with abnormal tertiary or quaternary structures. That grants them resistance (to some extent, at least) to proteases (1).

Also, researchers believe that prions are able to replicate (2), by changing the structure of other proteins.

Besides, It has been shown that small quantities of intact proteins do cross the gastrointestinal tract in animals and adult humans (3), and that this is a physiologically normal process required for antigen sampling by sub-epithelial immune tissue in the gut.

So, the resistance to the proteases and the ability to replicate in certain conditions might explain the odds of a prion crossing the gastrointestinal tract and infecting an individual.


References:

  1. http://www.ncbi.nlm.nih.gov/pubmed/24338008
  2. http://www.nejm.org/doi/full/10.1056/NEJM199609193351218
  3. http://www.ncbi.nlm.nih.gov/pubmed/3060169

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