After removal of the NH2 group, the amino acid becomes the corresponding keto acid.

rea level falls. On the other hand, it has been also shown that when the kidneys are removed (keeping the liver intact) the blood urea level rises. This proves that liver synthesizes urea but the kidneys eliminate it from the body. Utility The nitrogen of the amino acids are removed as urea. Normally a healthy adult person excretes about 15 gms of nitrogen per day, 95% of this nitrogen is excreted as urinary urea. As stated earlier, the NH2 group of the amino acids are ultimately (via the route waivoling glutamic acid deamination) removed as NH3. This NH3, as already stated, is highly toxic. NH3 thus formed, is utimately converted into urea. In liver failure urea synthesis stops and blood NH3 concentration rises causing damage of the brain. This ultimately may produce coma and death. As man and other mammals excrete nitrogen as urea, they are callei ireotelic' animals (N.B. urea is a non toxic substance). Birds and reptiles excrete nitrogen as uric acids and hence they are ailed 'uricotelic' animals. On the other hand, some marine formsof life excrete nitrogen chiefly as ammonia (N.B. It may be oted that the marine animals have maximum access to water so that the ammonia can be greatly diluted, whereas the birds ave minimal access to water In birds, the end product is uric acid which is water insoluble. Man and mammals occupy an itermedialed position) Source A molecule of urea contains two nitrogen atoms, one of which comes from the NH3 and the oher from the NH2 group of another ammo acid, the aspartic acid. Steps (Fig. 7.12.2) Steps of urea synthesis in the live were discovered in 1932 by Krebs and Hensehte and hence the entire process is called Krebs-Hensehte cycle. In short, the steps are as follows :1. NH3 (obtained from deamination of glutamic acid or coming from intestine as a result of bacteria ction) combines with CO2 (obtained from bicarbonate). This then combines with a phosphate radical (obtained from ATP). he result is formation of a compound, called carbamyl (also called carbamoyl) phosphate. The reaction is catalyzed by the nzyme, carbamoyl phosphate synthetase. 2. In the next step, the carbamoyl portion of carbamoyl phosphate is transferrei j ornithme to form citrulline. Thus;The reaction is catalyzed by the enzyme arginase which is found in the liver of at mammals but not in those of the birds or reptiles.. Fumaric acid is converted into oxaloacetic acid (OAA). Subsequently, the OAA, by the process of transmination can become asparlic acid again in the liver and thus recycled in the Krebs-Hensehte cycle. The whole cycle is sometimes called ornithme cycle. Clinical considerations. Uremia In renal failure, large scale erangements of body metabolites occur. This includes accumulation of waste products like urea. Normal blood urea oncentration is about 15 to 40 mgms/100 mi. In renal failure, blood urea rises and may attain such values like 200 mg/100 ml or so. Although urea is almost nontoxic, blood urea determination offers a valuable and at the same time an easy diag ostic and prognostic procedure, because the level of blood urea gives an idea of the intensity of the renal failure. For hysicochemical reasons, retention of urea is associated with retention of body water. Patients who have high blood urea alues, therefore, develop puffiness of the face. Renal failure is often known as uremia. The steps of Krebs-Henselite cycle may be represented as shown in fig.7.12.2. Decarboxylation The carboxyl radical, COOH may be removed from some amino acids forming the corresponding amines. The reactions are catalyzed by the enzyme decarboxylase and the process is ailed decarboxylation. The general reaction is, Some very important amines are formed in this way; examples are : (i istamine from histidine (for details see, local hormones, chap.6.8) (ii) 5 hydroxytryptamine or serotonin from 5 ydroxytryptophan which in turn is derived from tryptophan (see local hormones, chap.6.8) (iii) tyramine from tyrosine. (iv drenalin, (v) nor adrenalin and (vi) dopamine from 3,4 dihydroxyphenylalanine (DOPA) which in turn is derived from pheny alanine and so on. Decarboxylation of cysteine produces mercaptoethylamine which is a constintent part of coenzyme A 'pecific dynamic action (SDA) The 19th century German chemist Ely Rubner, the father of modern biochemistry am ntrition coined this term. Amino acids while undergoing catabolism produce an extra amount (extra = more than the value redicted on the basis of bomb calorinetri) of heat called specific dynamic action (SDA). Exact mechanism of SDA remains nclear but this much is known that (i) while amino acids are catabolized in the liver then the SDA is produced, and (ii) SDA is ue to the fact that metabolism is stimulated. SDA is produced not only by amino acids but also by glucose and fat of the iods. Because of the close interrelationship between SDA and nutrition, this issue has again been discussed in the chapte of nutrition (see chap. 7. 16). Fate of keto acids After removal of the NH2 group, the amino acid becomes the corresponding keto acid. This keto acid can merge into the stream of carbohydrate or fat metabolism. Accordingly, the ammo acids are o iree classes : Glucogenic amino acids The corresponding keto acid merges into the stream of glucose metabolism. A typics xample is Alanine, arginine, asparlic acid, cystine, glutamic acid, glycine, hydroxyproline, serine, valme, melhionine, proline ireonine and histidine are qlucoqenic amino acids. Ketoqenic amino acids correspondinq keto acid
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