METABOLISM OF INDIVIDUAL AMINO ACIDS

merges to the stream of fat metabolism. Leucine belongs to this class. Both ketogenic and glycogenic Isoleucine, lysine phenylalanine, tyrosine and tryptophan are in this class. METABOLISM OF INDIVIDUAL AMINO ACIDS As a rule amino acids are used for synthesis of structural proteins, enzymes, hormones etc. and when in excess, undergo catabohsm to yield urea and keto acids. However, some particular amino acids are required for synthesis of someparticular compounds of specific biological importance. In the following paragraphs metabolism of some special individual amino acids will be discussed. Glycine Glycine is required for the synthesis of (i) pyrrole, (ii) purine and (iii) creatine. Glycine is also required for synthesis of (iv) serine and (v) glutathione. Glycine can conjugate with cholic acid (bile acid) to form (vi) glycocholic acid. Glycine also conjugates with benzoic acid to form (vii) hippuric The reaction occurs in the liver. It is in this way, benzoic acid is detoxicated. Hippuric acid is excreted via urine. Benzoic acid is toxic, but hippuric acid is not. II. Phenyl alanine and tyrosin As shown in Table 7.11.2. phenyl alanine (a amino□phenyl propionic acid) and tyrosine (para hydroxy phenyl alanine) are closely related compounds. Phenyl alanine in our body can be converted into tyrosine but tyrosine cannot form phenyl alanine. Thus, phenyl alanine is an essential amino acid while tyrosine is not. The Fig. 7.12.2. Krebs urea cycle Argmosuccinic acid is also known as argminosuccinic acid ] conversion, is catalyzed by phenyl alanine hydroxylase (Fig. 7 .12. 3, site A). Fig. 7.12.3. The different possible fates of phenyl alanine and tyrosine. (The letters A, B and C indicate possible sites of blocks in diseases, see text) Tyrosine is required for the synthesis of (i) thyroid hormones, (ii) nor adrenalin (iii) adrenalin (adrenalin is methyl nor adrenalin), (iv) dopamine and (v) melanin. Catabolism of phenyl alanine is outlined in figure 7.12.3. In short, phenyl alanine can have one of the following two catabolic fates : (i) phenyl alanine may be deaminated, the alanine moiety thus becoming pyruvic acid (while the aromatic ring remaining unaltered) and thus pheny alanine becoming phenyl pyruvic acid. (ii) phenyl alanine D tyrosine (b hydroxy phenyl and aceto acetic acid can enter Krebs tricarboxylic acid cycle and completely catabolized into C02 and H20. Applied physiology; inborn erros of metabolism Pheny ketonuria In this condition, the enzyme phenyl alanine hydroxylase (A in fig. 7.12. 3) is deficient. This results in failure a conversion of phenyl alanine to tyrosine, phenyl alanine accumulates and is converted into phenyl pyruvic acid which is excreted via urine. The patients are mentally deficient. The mental deficiency is due to the fact that excess phenyl pyruvic acid depresses pyrunate dehydrogenase; this enzyme causes conversion of pyruvic acid of brain to acetyl CoA (active acetate), so that, ultimately Krebs tricarboxylic acid cycle in brain suffers in this disease. Albinism Melanin, the pigment of skin is normally present in all individuals. Colored persons (e.g., Negroes) have excessive melanin. Its function is to protect the individual from excessive and strong sun rays (see also, protective functions of the skin, chapter 8.5). Absence of enzyme, melanocyte tyrosinase leads to failure of melanin formation from tyrosine (B in fig. 7.12. 3) and the person develops, albinism. The skin of the affected region of the body is white; if the whole body is affected, the entire body is white and there is no pigment in the iris. The person has a strong aversion against exposure to sun. Another such inborn disease, very much allied to phenyl ketonuria, is, alkaptonuria, where there is deficiency of the enzyme, homogrntisic acid oxidase (C in 7.12. 3) resulting in failure to catabolize homogentisic acid. These subjects pass homogentisic acid through their urine, so that the urine on keeping becomes dark. There is , in these subjects, however, no mental retardation or neurological symptom. III. Tryptophan Tryptophan (a amino p indole propionic acid) is an essential amino acid containing an indole ring (Table 7.11.2). Apart from the fact that tryptophan is required for protein synthesis it is required in connection with some special compounds as follows: Tryptophan is the precursor of serotonin (5 hydroxytryptamine, for greater details, see local hormones, chap 6.8. There is an interesting relationship between tryptophan, nicotinic acid and pellagra. In short, nicotinic acid, a member of vitamin B Complex, can be synthesized from tryptophan (for details, see nicotmic acid, chapter 7.14). In the intestine; tryptophan can be converted into indole or skatole (indoxyl sulphate) which imparts its offensive smell to the stool. Indoxyl is also absorbed and ultimately excreted via urine as urinary indican. Sulphur containing amino acids Methionine (o ammo methyl thiol butyric acid), cys-teine (a amino □ thiol propionic acid) and eystine (= dicysteine) are the three amino acids which contain sulphur. Most of the food sulphur comes from these sulphur containing amino acids. Metabolism of sulphur containing compounds, therefore, practically means metabolism of sulphur containing amino acids. The CH3 radical, printed in thick letter, is called labile methyl group'. It is called labile, because it can be removed very easily. Subsequently it can be attached to a new compound to produce another compound. The CH3 group, it used to be previously. believed that, cannot be synthesized in our body. The statement is not altogether true. Little amount of methyl (CH3) gioups