From the previous discussion, it can be seen that the metabolism of most amino acids involves removal of the amino groups by transamination. 2-oxoglutarate is the main acceptor of these amino groups, being converted to glutamate, which can then be deaminated to release ammonium. However, ammonium is highly toxic and cannot be allowed to accumulate, so it is converted to urea, which is the form in which most of the nitrogen derived from protein is excreted from the body. Urea is formed in the liver by the cyclic series of reactions shown in Figure 10. It can be seen that only one of the nitrogen atoms in the urea molecule is actually derived from ammonium, via carbamyl phosphate. The other nitrogen atom comes from quinolinic acid
quinolinic acid ribonucleotide
nicotinic acid ribonucleotide
carbamyl phosphate ornithine
V H2O arginine citrulline ATP
^ aspartate argininosuccinate fumarate Figure 10 The urea cycle.
aspartic acid, which is formed by transamination of oxaloacetic acid.
The rate of production of urea by the liver is normally greater than the rate of urea excretion in the urine. This is because some of the urea diffuses into the colon, where it is hydrolyzed to ammonia by bacteria. The ammonia can be absorbed and taken up by the liver, where it can be reincorporated into amino acids, thereby augmenting the net supply of nonessential amino acids. The colonic bacteria can also use ammonia to synthesize essential amino acids, and there is evidence that some of these essential amino acids can also be absorbed and utilized by the human body. However, the rate at which this happens is clearly not sufficient to meet the body's requirements for essential amino acids.
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