It action in the acid contents of

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.

A second proteolytic enzyme with an optimum pH of 3*3 to 4 0 has recently been found.

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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

Inactive proenzyme – Active proteolytic enzyme

Proteins ———–H2O—a Proteoses———-H2O——a 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.

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.