Ammonium ions metabolism

The two ammonium ions produced from glutamine as illustrated in Figures 8.4 to 8.6 are secreted into the PCT lumen the by a Na+/H+ antiport (the NH4+ substitutes for H+). Subsequent metabolism of 2-oxoglutarate has the potential to generate two bicarbonate ions from the hydration of carbon dioxide by carbonic anhydrase ... 

Ammonia can diffuse freely into the urine through the tubule membrane, while the ammonium ions that are formed in the urine are charged and can no longer return to the cell. Acidic urine therefore promotes ammonia excretion, which is normally 30-50 mmol per day. In metabolic acidosis (e.g., during fasting or in diabetes mellitus), after a certain time increased induction of glutaminase occurs in the kidneys, resulting in increased NH3 excretion. This in turn promotes H"" release and thus counteracts the acidosis. By contrast, when the plasma pH value shifts towards alkaline values alkalosis), renal excretion of ammonia is reduced. 

Secretion or separation of metabolic products and by-products For example, organic acids such as lactate and pyruvate, or other compounds hke CO2, ammonium ions, and H2S are secreted as by-products. 

Reduction of nitrite denitrification. The nitrite formed in Eq. 18-25 is usually reduced further to ammonium ions (Eq. 18-27). Tire reaction may not be important to the energy metabolism of the bacteria, but it provides NH4+ for biosynthesis. This six-electron reduction is catalyzed by a hexaheme protein containing six c-type hemes bound to a single 63-kDa polypeptide chain.336,337... 

McQueen A, Bailey E (1990), Effect of ammonium ion and extracellular pH on hybridoma cell metabolism and antibody production, Biotechnol. Bioeng. 35 897-907. 

Pharmacokinetics Methenamine is orally administered. In addition to formaldehyde, ammonium ion is produced in the bladder. Because the liver rapidly metabolizes ammonia to form urea, methenamine is contraindicated in patients with hepatic insufficiency, in which elevated levels of circulating ammonium ions would be toxic to the CNS. Methenamine is distributed throughout the body fluids, but no decomposition of the drug occurs at pH 7.4 thus, systemic toxicity does not occur. The drug is eliminated in the urine. 

There have been many anecdotal reports of metabolic acidosis in glue sniffers, attributed to renal tubular acidosis. However, it has been suggested that it is in fact due to overproduction of hippuric acid resulting from the metabolism of toluene, with or without a reduced rate of urinary ammonium ion excretion (32). 

Amino acids ate not the only source of ammonium ions produced in the body. Much of the ammonia produced, especially in the brain, arises from the hydrolysis of purines. Adenylate deaminase catalyzes the hydrolysis of AMP, yielding IMP and ammonium ions Cooper and Plum, 1987). IMP is inosine monophosphate (inosinic acid), GMP may also be hydrolyzed in this manner, yielding xanthosine and ammonium ions. Further details on purine metabolism occur at the end of this chapter and under Folate in Chapter 9. 

Within the modern nitrogen cycle there is a variety of different metabolic processes, which give rise to a range of isotopic fractionations. These are summarized in Fig. 6.1 (after Papineau et al., 2005). Here we focus on two specific processes within the nitrogen cycle. First, is the production of ammonium ions, and second the process of denitrification. 

Ammonium ions are produced by two different types of metabolic process. One is via the bacterial fixation of nitrogen. The other is... 

Two types of reactions play prominent roles in amino acid metabolism. In transamination reactions, new amino acids are produced when a-amino groups are transferred from donor a-amino acids to acceptor a-keto acids. Because transamination reactions are reversible, they play an important role in both amino acid synthesis and degradation. Ammonium ions or the amide nitrogen of glutamine can also be directly incorporated into amino acids and eventually other metabolites. 

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