Proteins waste nitrogen

Toxicity and Tacfcaging of Waste Nitrrigen Illustrated bv Experintental Nutrition Regulation of Protein Catabolism Nutritional Aspeets of Protein CaiabolUiti Nitrogen Balance C.rowth... 

Toxicity and Packaging of Waste Nitrogen Illustrated by Experimental Nutrition Regulation of Protein Catabolism Nutritional Aspects of Protein Catabolism Nitrogen Balance Growth... 

FIGURE 8.20 Elimination of ammonium ions via the urea cycle or via direct excretion. With protein catabolism, the excretion of waste nitrogen via the urea cycle results in net production of acid in the body however, excretion of ammonium ions by the kidney into the urine does not result in this production of acid in the body. 

Additionally, blood is the medium for exchange of nutrients and waste products. Nutrients, such as the polar sugar glucose, enter the blood from the intestine or the liver. Because glucose molecules are polar, they dissolve in body fluids and are circulated to tissues throughout the body. As noted above, nonpolar nutrients are transported with the help of carrier proteins. Similarly, nitrogen-containing waste products, such as urea, are passed from cells to the blood. They are continuously and efficiently removed from the blood by the kidneys. 

The primary function of the urea cycle is to rid the body of waste nitrogen. Deficiency in the activity of any of the six enzymes in the urea cycle may result in the accumulation of ammonia, often to toxic concentrations. Treatment involves restricting protein, preventing catabolism, supplementing amino acids that are normally produced by the urea cycle, and promoting the excretion of nitrogen via alternative pathways. Outcomes are guarded and appear to be better for patients identified by NBS compared to patients identified clinically. Liver transplantation is a treatment option, especially for patients with a severe form of the disorder. 

The need for protein in the diet was demonstrated early in the nineteenth century, when it was shown that animals which were fed only on fats, carbohydrates and mineral salts were unable to maintain their body weight and showed severe wasting of muscle and other tissues. It was known that proteins contain nitrogen (mainly in the amino groups of their constituent amino acids section 6.4.1), and methods of measuring total amounts of nitrogenous compounds in foods and excreta were soon developed. 

Nutritional considerations. The diet should be high in carbohydrates, low in fst, and supplemented with limited amounts of high-quality proteins that reduce the need for waste nitrogen metabolism. Use of drugs or supplements that requite liver metabolism should be avoided. 

Blood urea nitrogen (BUN) A waste product in the blood produced from the breakdown of dietary proteins. The kidneys filter blood to remove urea and maintain homeostasis. A decline in kidney function results in an increase in BUN. 

The alanine cycle accomplishes the same thing as the Cori cycle, except with an add-on feature (Fig. 17-11). Under conditions under which muscle is degrading protein (fasting, starvation, exhaustion), muscle must get rid of excess carbon waste (lactate and pyruvate) but also nitrogen waste from the metabolism of amino acids. Muscle (and other tissues) removes amino groups from amino acids by transamination with a 2-keto acid such as pyruvate (oxaloacetate is the other common 2-keto acid). 

Urea cycle This pathway converts ammonia, a toxic nitrogen-containing waste product of protein metabolism, into another, less toxic molecule called urea, which can be eliminated from the body as urine. 

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