Gastric juice and enzymes in animals

Gastric juice and enzymes in animals

Gastric juice

Gastric juice is a colorless fluid, containing HCl, enzymes and mucus. Concentration of HCI in the stomach content varies with the nature of food, stage of digestion, amount of saliva to be neutralized etc.

Hydrochloric acid

1. HCl is actively secreted by the parietal/oxyntic cells of the fundic glands involves expenditure of energy, which is derived from the oxidative metabolism.  
2. Diffusion of CO₂ from the plasma into parietal cells
3. Higher concentration of carbonic anhydrase in the parietal cells
4. Chemical reaction of CO₂ with cellular water in the presence of “carbonic anhydrase”
5. Formation of carbonic acid ( H₂CO₃ ) in the parietal cells
6. Dissociation of H₂CO₃ in to HCO₃^– + H⁺
7. Diffusion of HCO₃^– ions into plasma with a exchange of Cl^– ion from the plasma
8. During active digestion, the concentration of HCO₃ increases very high and increases the pH of blood, called as alkaline tide.
9. Active exchange of H⁺ and K⁺ ions between parietal cell and luminal surface of the stomach by of H⁺ – K⁺ ATPase, enzyme present in the luminal surface of the stomach
10. H⁺ ion is actively secreted into the luminal surface of the parietal cell with a exchange of K⁺ ions into the parietal cell
11. Reaction of K⁺ ions with Cl^– ion and formation of KCl in the parietal cell
12. Release of KCl into the luminal surface
13. Reaction of KCl with H⁺ ion and the formation of K⁺ ion and HCl

Functions of HCl

1. Activates pepsinogen, the inactive form of enzyme into active pepsin, hydrolysing enzyme of protein
2. Slight hydrolysis of sucrose
3. Also function as antiseptic in the stomach.

Gastric enzymes

Pepsin

• Proteolytic enzyme, involved in hydrolysis of proteins into polypeptides
• Synthesized as “Pepsinogen” (inactive form of pepsin)
• Pepsinogen is activated by HCl 
• Optimum pH of 1.5 – 3 is required for pepsin activity

Rennin(Chymosin, Rennet)

• Rennin is a milk coagulating enzyme, present in young animals(Calf, Lamb, piglet etc.,)
• Secreted as prorennin (inactive form of rennin)
• Prorennin is activated by HCl
• Rennin reacts with casein (Milk protein)  in the presence of calcium ion forms calcium paracaseinate to delay the passage of milk through stomach for prolonged action of pepsin on casein 

Gastric lipase

• Gastric lipase acts on emulsified fat
• Hydrolysis fat in to fatty acids and glycerol
• Require the optimum pH of 5.5 – 7.5
• Its concentration in carnivores is low
• Absent in birds and ruminants.