Tryptophan and Histidine

Huvaere, K., and Skibsted, L. H. (2009) Light-Induced Oxidation of Tryptophan and Histidine. Reactivity of Aromatic N-Heterocycles toward Triplet-Excited Flavins, Journal of American and Chemical Society, Vol. 131, (May 2009) pp. 8049-8060, ISSN 0002-7863. 

Dopamine is the decarboxylation product of DOPA, dihydroxyphenylalanine, and is formed in a reaction catalysed by DOPA decarboxylase. This enzyme is sometimes referred to as aromatic amino acid decarboxylase, since it is relatively non-specific in its action and can catalyse decarboxylation of other aromatic amino acids, e.g. tryptophan and histidine. DOPA is itself derived by aromatic hydroxylation of tyrosine, using tetrahydrobiopterin (a pteridine derivative see Section 11.9.2) as cofactor. 

Dopamine /S-hydroxylase adsorbed on gold electrode has been investigated elec-trochemically and applying quartz crystal microbalance [198]. In the neutral phosphate buffer solution, the adsorbed layer was stable and did not desorb within the potential range of 0.6 to —0.7 V (versus AglAgCl —1 M KCl). At potentials more positive than 0.8 V, the adsorbed compound was oxidized and, probably, residual tyrosine, tryptophan, and histidine participated in this process. 

Among the essential amino acids, the aromatic amino acids (phenylalanine, tyrosine, and tryptophan) form by a pathway in which chorismate occupies a key branch point. Phosphoribosyl pyrophosphate is a precursor of tryptophan and histidine. The pathway to histidine is interconnected with the purine synthetic pathway Tyrosine can also be formed by hydroxylation of phenylalanine (and thus is considered conditionally essential). The pathways for the other essential amino acids are complex. 

Phosphoribosyl pyrophosphate (PRPP) is important in both, and in these pathways the structure of ribose is retained in the product nucleotide, in contrast to its fate in the tryptophan and histidine biosynthetic pathways discussed earlier. An amino acid is an important precursor in each type of pathway glycine for purines and aspartate for pyrimidines. Glutamine again is the most important source of amino groups—in five different steps in the de novo pathways. Aspartate is also used as the source of an amino group in the purine pathways, in two steps. 

Electron beam and 7-ray irradiation of ground beef have been compared with regard to amino acid destruction. The most sensitive acid to irradiation was cystine, followed by tryptophan and histidine. The greatest amounts of destruction were obtained with 24 m.e.v., 200 juamps electron beam irradiation. Electron beam density appeared to be as important as total irradiation dose in destroying amino acids. In general, little damage to amino acids and thus to the nutritive value of beef was produced by irradiation. 

To study the effect of the Maillard reaction on nutritive value of protein, Patton, et a l. (57) heated purified casein and soybean globulin in 57. glucose solution for 24 hrs at 96.5°C, and found significant losses of lysine, arginine, tryptophan, and histidine (52). 

This reaction is catalyzed by manganese ions at pH values from 6 to 7.5. S02 can also react with cystine to yield a series of oxidation products. Some of the possible reaction products resulting from the oxidation of sulfur amino acids are listed in Table 3-11. Nielsen et al. (1985) studied the reactions between protein-bound amino acids and oxidizing lipids. Significant losses occurred of the amino acids lysine, tryptophan, and histidine. Methionine was extensively oxidized to its sulfoxide. Increasing water activity increased losses of lysine and tryptophan but had no effect on methionine oxidation. 

Oxaline (143) was isolated in 1974 by Nagel et al. (183,184) from cultures of the toxicogenic fungus Penicillium oxalicum. The compound may be classified as an indole alkaloid, but it is one of the three indole alkaloids known at present that also contains an imidazole substituent the other indole alkaloids being roquefortine (144) and neoxaline (145). The structure of oxaline was deduced from physicochemical data and confirmed by single-crystal X-ray analysis. It has been suggested that in the biosynthesis of oxaline, nature makes use of the amino acids tryptophan and histidine (184). 

The biosynthesis of roquefortine was investigated by feeding labeled mevalonic acid lactone, tryptophan, and histidine to Penicillium roqueforti these compounds were incorporated (190,191). A biogenetic pathway for roquefortine obtained from Stilton cheese has been suggested (191). 

XII, there is no strong competition by simple amino acids or by hydroxy-amino acids in acidic media. A more rapid reaction is observed with the basic amino acids, phenylalanine and aspartic acid, and a very rapid consumption of NBS by the sulfur amino acids, tryptophan and histidine. 

The awareness of the many advantages of this modification reaction should be matched by the knowledge of the numerous side-reactions observed to result from the reaction of TNM with proteins. In addition to nitration of tyrosine, the following side-reactions have been reported in several (but not all) proteins studied (1) inter- and intramolecular cross-linking, (2) oxidation of sulfhydryl groups to a variety of products, (3) oxidation of methionine, (4) modification of tryptophan and histidine, (5) modification of prosthetic groups. It is apparent therefore that a successful application of TNM to the selective modification of tyrosine is achieved in those cases where an unusually rapid reaction... 

The interesting diazonium sulphate betaine (495) has been prepared by diazotisation of the corresponding amino-sulphate and shown to couple under physiological conditions with cysteine, tryptophan, and histidine. 

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