Fate of the Amino-Acids
In the gastro-intestinal tract the complex food proteins, under the hydrolytic action of enzymes, break down into a variety of substances, all of which belong to the group of a-amino-acids. These same absorbed from thence into the blood are transported to the various organs and tissues. Arrived thither the amino-acids are subjected to a process of sifting. Thus some are invoked for the reconstruction of broken down proteins, i.e., are re-synthesised into the body’s own characteristic tissues.
The surplus amino-acids, viz., those not required for purposes of cell repair, undergo deaminisation. Two residues then result, one represented by ammonia, and the other by the remaining relics of the amino-acid molecule. The former is excreted as urea and the latter is oxidised to produce energy.
But there is yet another source of amino-acids, viz., the disintegration of tissue protein. To this end almost all bodily tissues possess intracellular enzymes capable of converting their proteins into the same simple products from which they took origin.
Comparably with those of exogenous origin, these amino-acids of endogenous formation undergo a like deaminisation; in other words, the bulk of their carbon, oxygen, and hydrogen is oxidised to form CO₂ and water, the residue combining with nitrogen to form urea, etc.
The main end-product, then, of protein metabolism is urea, with traces of its forerunner ammonia. But there are also other waste nitrogenous metabolites. Thus, of the various amino-acids that become built up into tissue protein, some subsequently break down into products not belonging to the amino-acid category, viz., creatine and creatinine. Some of the amino-acids, too, are excreted unchanged in the urine. Lastly, to these must be added those closely related substances, the purin bodies, the end-products of nuclein as opposed to general protein metabolism, of which latter urea is the terminal product. To sum up, in a man on ordinary diet about 90 per cent. of his total nitrogen is excreted as urea, about 3 per cent. as ammonia, the residue of the nitrogen appearing in the form of other nitrogenous metabolites.