Glutamine proton excretion

Generation of ammonia from glutamine and its excretion as NH4 is an important mechanism for elimination of protons, particularly during severe metabolic acidosis, when it becomes a significant mode of nitrogen excretion. Most renal glutamine is derived from muscle (Chapters 17 and 22). Glutamine provides two molecules of NH3 ... 

Fig. 42.3. Interorgan amino acid exchange after an overnight fast. After an overnight fast (the postabsorptive state), the utilization of amino acids for protein synthesis, for fuels, and for the synthesis of essential functional compounds continues. The free amino acid pool is supported largely by net degradation of skeletal muscle protein. Glutamine and alanine serve as amino group carriers from skeletal muscle to other tissues. Glutamine brings NH4 to the kidney for the excretion of protons and serves as a fuel for the kidney, gut, and cells of the immune system. Alanine transfers amino groups from skeletal muscle, the kidney, and the gut to the liver, where they are converted to urea for excretion. The brain continues to use amino acids for neurotransmitter synthesis.

In general, it can be stated that acids, applied as acids or synthesized endogenously, do not influence the acid—base balance if they are eliminated unchanged as acids. However, they exert a clear influence on the plasma bicarbonate if they are excreted, renally or extrarenally, in the ionized form as anions, leaving their proton behind in the extracellular fluid. Examples of exogenously applicable, urine acidifying substances are hydrochloric and phosphoric acid, ammonium chloride, lysine and glutamine hydrochloride, and calcium chloride. The two mineral acids are excreted as chloride and phosphate anions, respectively. The ammonium ion of ammonium chloride is eliminated as neutral... 

An important function of the kidney in regulating the acid-base balance is ammonium secretion. Ammonia probably forms in the collecting tubes. The tubule cells form ammonia by deaminating glutamine. The ammonia reacts with protons to yield ammonium ions. It is not known whether these ammonium ions are formed within the tubular cell or within the tubular fluid. In any event, the trapping of protons by ammonia and the excretion of the ammonium ion into the tubular fluid confer buffering properties to the tubular fluids, reduce the difference between the pH of the tubular and the intracellular fluid, and further facilitate the excretion of hydrogen ions. 

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