Case study- Achlorhydria (Discussion)

A 65- year-old woman is evaluated by a gastroenterologist for progressive signs and symptoms of malnutrition. Throughout her work up, she is found to have significantly decreased stomach acid. She is also found to have antibodies to gastric parietal cells, which are normally responsible for the production of acid.

Why is hydrochloric acid important in digestion?

A. It stimulates the cleavage of trypsinogen to trypsin

B. It is required for the activity of α- Amylase

C. It converts pepsinogen to pepsin

D. It is required for lipid digestion

E. It drives the secondary transport of amino acids

Answer- The correct answer is-C- it converts pepsinogen to pepsin.

The patient has presented with Achlorhydria and such patients have difficulty in protein digestion as well as absorption of vitamin B12, as the same parietal cells synthesize Intrinsic factor which is required for the absorption of vitamin B12.

Protein digestion begins in the stomach, where the acidic environment favors protein denaturation. Gastric juice is a unique solution containing hydrochloric acid and the proteolytic enzymes. Gastric HCl is too dilute (pH 2 to 3) to hydrolyze proteins. The acid functions to kill some bacteria and to denature proteins, thus making them more susceptible to subsequent hydrolysis by proteases.

The primary proteolytic enzyme of the stomach is pepsin, a nonspecific protease that, remarkably, is maximally active at pH 2. Thus, pepsin can be active in the highly acidic environment of the stomach, even though other proteins undergo denaturation there. This acid stable endopeptidase is secreted by the serous cells of stomach as an active zymogen (Pepsinogen). Pepsinogen is activated to Pepsin by HCl or auto catalytically by other pepsin molecules that have already been activated. Pepsin releases peptides and some free amino acids.

The conversion of trypsinogen to trypsin is brought about auto catalytically and primarily by Enterokinase, the enzyme secreted by the intestinal cells.

HCl is not required for the activity of Alpha amylase. Alpha amylase is a Metalloenzyme, it requires calcium for its activity. Amylases catalyze the hydrolysis of starch. Alpha amylase makes random cuts whereas Beta amylase, the other form of enzyme, secreted by bacteria, fungi and plants, starts its action from the non reducing ends.

HCl is not required for lipid digestion and its is also not required for the secondary transport of amino acids. The secondary transport of amino acids is an energy requiring process and takes place with the help of a carrier protein; HCl has no role in this process.

In fact all the enzymes of digestion except for gastric enzymes are acid labile, they are denatured in the presence of HCl. Amylases, lipases and the pancreatic or intestinal proteolytic enzymes become inactive in the acidic medium, they work best in the alkaline pH and bicarbonates present in pancreatic juice help in maintaining the optimum pH for the action of these enzyme.

Out of the given options, Pepsinogen is the only acid stable enzyme, hence it is the correct answer.

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