Nitrogen Metabolism Nitrogen Metabolism Most of the nitrogen in the diet is consumed in the form of protein. Dietary amino acids are constantly required for the replacement of body proteins. Disposable of excess nitrogen is a major component of nitrogen metabolism. Urea is the main nitrogenous product excreted by terrestrial mammals. Urea is non-toxic and has very high water solubility.
Gastrin stimulates the parietal cells to secrete HCl and the chief cells to secrete pepsinogen.
Pepsin is secreted as the inactive zymogen, pepsinogen. Pepsinogen is activated at low pH to cleave 42 amino acids from its N-terminus to make pepsin. Pepsin cleaves after Phe, trp and Tyr. Pepsin releases peptides and a few free amino acids. As the stomach acid enters the small intestine, the low pH triggers the secretion of secretin in the blood.