The Formation of Urea
The question as to whether urea, the end-product of general nitrogenous catabolism, was derived from the amino-acids, brought in the portal blood to the liver, was for long a vexed one. This because the earlier attempts to detect amino-acids in the portal blood, during the digestion of copious amounts of protein, proved futile. On the other hand, the same workers found that free ammonia was present in greater amounts in the portal vein than in the systemic circulation.
This, to their mind, seemed to indicate that the amino-acids, during their passage through the intestinal mucous membrane, underwent deaminisation. According to this view the ammonia, thus split off from the amino-acids, was the precursor of urea.
But the claim that more free ammonia was present in the portal vein than in the systemic circulation was disproved by Folin and Denis. Invoking more delicate methods of hæmo-analysis, they found that the amount of ammonia and urea in the portal blood was not increased during the absorption of amino-acids from the lumen of the intestine. Moreover, they found that the ammonia present was of minimal amount, produced in the main by putrefactive bacteria. Lastly, they discovered that amino-acids were actually present in the portal blood.