How exactly is casein digested?

Question

I mean it seems first step is rennin or pepsin digestion in stomach - then what happens with remaining peptides? I am interested in the whole process from casein to amino acids. Is there brush border enzymes involved in the small intestine or all process is in the stomach?

Answer 1

1. Casein proteins are soluble in milk but form insoluble curds once they reach the stomach, making it hard for digestive enzymes to break them apart. Slower digestion also is associated with delayed release of the protein's amino acids into the bloodstream.

milkgenomics.org/article/dairy-protein-digestion-life-slow-lane/

These properties of milk caseins may have evolved to benefit mammalian infants, but slowed digestion offers the benefit of increased satiety to milk drinkers of all ages. Eating foods, like milk, that take the slow lane may mean eating less food overall.

Casein proteins in milk form small spheres, called micelles, with the hydrophilic (water-loving) portions of the protein on the outside of the sphere and the hydrophobic (water-fearing) portions on the inside. With hydrophilic structures on the outside, the micelles are soluble in water (or milk, which is mostly water) [1,2].

But when the micelles reach the stomach, “one of the most ingenious events in nature takes place” [2]. The digestive enzyme chymosin snips one of the bonds on the exterior protein (known as the kappa subunit), leaving only the hydrophobic subunits inside [1]. Without their protective layer, the now insoluble proteins form a curd.

2. It looks like various proteolytic enzymes digest the casein 'curds' in the stomach and intestine. "Human proteolytic enzymes were obtained in the activated state by collecting human gastric (HGJ) and duodenal juice (HDJ)" This is the article: In vitro digestion of bovine and caprine milk by human gastric and duodenal enzymes

3. Dipeptidyl Peptidase IV's (DPP IV), a type of protease, has been shown to break down the proteins in gluten and casein. https://www.livestrong.com › Diseases and Conditions

It's in our intestine: https://www.ncbi.nlm.nih.gov/pubmed/7522442

4. Brush Border Enzymes & casein [that one gives me your stackexchange question!] This article: "Effect of diet on enzymes of the brush border of the small intestine and kidney of rats" says: "Diets with low starch and high casein contents resulted in higher specific activity of leucineaminopeptidase in the small intestine than diets with high starch and low casein contents." You can find out a bit more about that enzyme by reading the abstract of the article.

5. 'Digestion of peptides in small intestine' in Google gives this: "The brush border of the small intestine is equipped with a family of peptidases. ... They function to further the hydrolysis of lumenal peptides, converting them to free amino acids and very small peptides. These endproducts of digestion, formed on the surface of the enterocyte, are ready for absorption."

www.vivo.colostate.edu/hbooks/pathphys/digestion/smallgut/absorb_aacids.html

[Hint - it's google searches all the way, with an occasional scholar.google search. ... Maybe I should get back to my own work and let you all do the searches!]