1. Pepsin:
It is the proteolytic enzyme of gastric juice secreted by the peptic cells (Chief cells) of the fundus part of stomach in the form of an inactive precursor or zymogen what is known as pepsinogen, which, on coming into contact with hydrochloric acid, is converted into active pepsin.
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It is an usual enzyme in that its optimum pH is in the range 1-6 to 2-4 and it, therefore, meets conditions most suited to its action in the acid contents of the stomach.
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A second proteolytic enzyme with an optimum pH of 3*3 to 4 0 has recently been found.
Although pepsin is a powerful proteolytic enzyme, it does not digest proteins to a-amino-acids, but instead it breaks down proteins to the stage of proteose or peptones, that is to say, it breaks the protein chain into shorter fragments but it does not liberate free amino- acids.
The products of peptic digestion (e.g., proteoses and peptones)- are later broken down to amino-acids by the proteolytic enzymes of the pancreas and intestinal juices. The entire peptic reaction can be summarized as follows:
(i) Pepsinogen –HCL, activator—- > Pepsin
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Inactive proenzyme – Active proteolytic enzyme
Proteins ———–H2O—à Proteoses———-H2O——à Peptones
(Digestive action of pepsin)
2. Rennin:
The stomach of the calf and other young animals including man, contains an enzyme what is known as rennin.
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It is milk curdling or protein-coagulating enzyme (proteinase) secreted as inactive prorennin which on coming in contact with hydrochloric acid is converted into active rennin.
It, in the optimum pH (5-6), acts upon the casein of milk to form first a soluble paracasein and a peptone like body.
In the presence of calcium ions there is then formed an insoluble calcium paracaseinate which separates out as a curd that can be digested by pepsin. The entire reaction can be summarized as follows:
(i) Prorennin –HCL—– > Rennin
Inactive proenzyme Active enzyme
(ii) Rennin-f-Casein-> Paracasein
(iii) Paracasein4-Ca-*Calcium paracaseinate (curdles)
(i) Calcium paracaseinate (curdle)-fPepsin ->Proteoses, peptones, polypeptides.
3. Gastric lipase:
It is a weak fat-splitting enzyme which acts in an acid medium, unlike other lipolytic enzymes. It is probably secreted by Chief cells of the fundic glands and is destroyed by alkalies.
The digestion of fat in the stomach is, however, of slight importance as it is completed in the intestine through the action of the lipase of pancreatic juice.