Amino acids gastric absorption

FIGURE 29-2. Levodopa absorption and metabolism. Levodopa is absorbed in the small intestine and is distributed into the plasma and brain compartments by an active transport mechanism. Levodopa is metabolized by dopa decarboxylase, monoamine oxidase, and catechol-O-methyltransferase. Carbidopa does not cross the blood-brain barrier. Large, neutral amino acids in food compete with levodopa for intestinal absorption (transport across gut endothelium to plasma). They also compete for transport across the brain (plasma compartment to brain compartment). Food and anticholinergics delay gastric emptying resulting in levodopa degradation in the stomach and a decreased amount of levodopa absorbed. If the interaction becomes a problem, administer levodopa 30 minutes before or 60 minutes after meals. 

Levodopa, a dopamine precursor, is the most effective agent for PD. Patients experience a 40% to 50% improvement in motor function. It is absorbed in the small intestine and peaks in the plasma in 30 to 120 minutes. A stomach with excess acid, food, or anticholinergic medications will delay gastric emptying time and decrease the amount of levodopa absorbed. Antacids decrease stomach acidity and improve levodopa absorption. Levodopa requires active transport by a large, neutral amino acid transporter protein from the small intestine into the plasma and from the plasma across the blood-brain barrier into the brain (Fig. 29-2). Levodopa competes with other amino acids, such as those contained in food, for this transport mechanism. Thus, in advanced disease, adjusting the timing of protein-rich meals in relationship to levodopa doses may be helpful. Levodopa also binds to iron supplements and administration of these should be spaced by at least 2 hours from the levodopa dose.1,8,16,25... 

In general, ethanol in low to moderate amounts, is relatively benign to most body systems. A moderate amount of ethanol causes peripheral vasodilation, especially of cutaneous vessels, and stimulates the secretion of salivary and gastric fluids the latter action may aid digestion. On the other hand, ethanol consumption in high concentrations, as found in undiluted spirits, can induce hemorrhagic lesions in the duodenum, inhibit intestinal brush border enzymes, inhibit the uptake of amino acids, and limit the absorption of vitamins and minerals. In addition, ethanol can reduce blood testosterone levels, resulting in sexual dysfunction. 

Copper absorption mainly occurs in the small intestine, although gastric uptake has been shown to occur to a smaller extent. Some copper may also be incorporated by inhalation and skin absorption. The extent of intestinal copper absorption varies with dietary copper content and is around 50% at low copper intakes (<1 mg Cu per day) but only 20% at higher intakes (>5mg Cu per day). Copper intestinal uptake is pH dependent and relatively efficient. Absorption is reduced by other dietary components, such as zinc (via metallothionein), molybdate, and iron, and increased by amino acids and by dietary sodium. ... 

The total quantity of fluid absorbed each day by the gut is estimated to be about 9 L, which is composed of 2 L oral intake, 1.5 L saliva, 2.5 L gastric juice, 0.5 L bile, 1.5 L pancreatic juice, and 1 L intestinal secretions. More than 90% of this fluid is absorbed in the small intestine. The maximal absorptive capacity for fluid is probably at least 20 L. Several hrmdred grams of carbohydrates, about 100 g of fat, and 50 to 100 g of amino acids are absorbed daily in the small gut, but maximal absorptive capacity is believed to be at least 10 times greater. This considerable reserve capacity may compensate for mild to moderate degrees of dysfunction induced by disease processes, at least in the early phases. The efficiency of absorption is due to the unique features of the... 

Some of the other functions which utilize active transport are (1) the secretion of saliva, gastric juice, and pancreatic juice during digestion (2) absorption of glucose and amino acids from the intestine (3) accumulation of calcium, manganese, magnesium, and phosphate ions within the mitochondria of cells and (4) reabsorption of glucose and amino acids from the kidney tubules. 

The ready equilibration between free amine and ammonium salt is important for the formulation and activity of amine-based pharmaceuticals. The neutral forms are typically insoluble in water, and therefore the medications are sold in their salt forms, to allow for ready oral or intravenous administration. The pK of the amino function also affects the distribution, metabolism, exaction, and binding to receptor sites. Thus, after ingestion, the acidic environment of the stomach (pH 2-4) will keep the drug protonated, therefore minimizing absorption through the nonpolar gastric walls. Once in the intestines (pH 5-7), the acid-base equilibrium... 

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