Protons excretion

Only the kidney is capable of excreting protons in exchange for Na" ions (see p. 326). In... 

Along with the lungs, the kidneys are particularly involved in keeping the pH value of the extracellular fluid constant (see p. 288). The contribution made by the kidneys particularly involves resorbing HCOs and actively excreting protons. 

The retro-Claisen product 12 is further converted into acetic acid which together with 13 exerts acid stress what results in an increased energy demand in order to excrete protons from the cell and in this way concomitantly affects NADH and ATP concentration in the cell [65]. 

The synthesis described met some difficulties. D-Valyl-L-prolyl resin was found to undergo intramolecular aminoiysis during the coupling step with DCC. 70< o of the dipeptide was cleaved from the polymer, and the diketopiperazine of D-valyl-L-proline was excreted into solution. The reaction was catalyzed by small amounts of acetic acid and inhibited by a higher concentration (protonation of amine). This side-reaction can be suppressed by adding the DCC prior to the carboxyl component. In this way, the carboxyl component is "consumed immediately to form the DCC adduct and cannot catalyze the cyclization. 

Root products are all the substances produced by roots and released into the rhizo.sphere (Table 2) (17). Although most root products are C compounds, they include ions, sometimes O, and even water. Root products may also be classified on the basis of whether they have either a perceived functional role (excretions and secretions) or a nonfunctional role (diffusates and root debris). Excretions are deemed to facilitate internal metabolism, such as respiration, while secretions are deemed to facilitate external proces.ses, such as nutrient acquisition. Both excretion and secretion require energy, and some exudates may act as either. For example, protons derived from CO2 production in respiration are deemed excretions, while those derived from an organic acid involved in nutrient acquisition are deemed secretions. 

G. Neumann and V. Romheld, Root excretion of carboxylic acids and protons in phosphorus-deficient plants. Plant Soil 2// 121 (1999). 

Protonated THAM (with CP or HCO, ) is excreted in the urine at a rate that is slightly higher than creatinine clearance. As such, THAM augments the buffering capacity of the blood without generating excess C02. THAM is less effective in patients with renal failure and toxicities may include hyperkalemia, hypoglycemia, and possible respiratory depression. 

The excretion of water soluble waste via the kidneys requires filtration followed by selective reabsorption from and secretion into the renal tubules. Regulation of normal blood pH within very strict limits due to proton secretion and bicarbonate reabsorption is a major role of the kidney. 

Additionally, the incomplete oxidation of fuel generates weak organic acid anions. Furthermore, there is loss of base in the faeces each day which, in effect, leaves the body with an excess of protons to be excreted. In total, about 70 milliequivalents (mEq) of acid require excretion each day. Note mEq is used to quantify the acid load because this takes account of the valency of the ion 1 mmol of sulfate, S04, for example, is 2 mEq of negative charge, requiring 2 mEq of protons for neutralization, but for monovalent ions, such as protons or bicarbonate, 1 mEq= 1 mmol. ... 

Since these are fundamentally important processes, the transport of protons into the glomerular filtrate and the transport of ammonia can be considered as secretions rather than excretions. 

As indicated above, one function of the kidney is to maintain the pH of the blood by excreting (secreting) protons into the glomerular filtrate during acidosis. In order to buffer these protons in the glomerular filtrate, ammonia is also excreted. The buffering occurs as follows ... 

Ammonia buffers protons in the tubules of the kidney to prevent formation of an acidic urine, when the kidney excretes (secretes) protons to control the pH of the blood. 

Urea (H2N-CO-NH2) is the diamide of carbonic acid. In contrast to ammonia, it is neutral and therefore relatively non-toxic. The reason for the lack of basicity is the molecule s mesomeric characteristics. The free electron pairs of the two nitrogen atoms are delocalized over the whole structure, and are therefore no longer able to bind protons. As a small, uncharged molecule, urea is able to cross biological membranes easily. In addition, it is easily transported in the blood and excreted in the urine. 

Approximately 60 mmol of protons are excreted with the urine every day. Buffering systems in the urine catch a large proportion of the H"" ions, so that the urine only becomes weakly acidic (down to about pH 4.8). 

When carbonic anhydrase inhibitors block the enzyme in the kidney, HjCOj formation— and consequently the availability of H3O+ (i.e., protons)—decreases. Since the Na+ ions in the filtrate cannot be exchanged, sodium is excreted, together with large amounts of water, as a result of ion hydration and osmotic effects. The result is diuresis, accompanied by a dramatic increase in urine volume. There is also failure to remove HCOj" ions because there is no H3O+ to form HjCOj, which would decompose to COj -1- HjO. Therefore, the normally slightly acidic urine becomes alkaline. The strong carbonic anhydrase inhibitors also increase K+ excretion, an undesirable effect. 

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