New reaction of O-benzoquinone at the thioether group of methionine

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

Reactivities of Quinone Methides versus o-Quinones in Catecholamine Metabolism and Eumelanin Biosynthesis

Melanin is an important biopolymeric pigment produced in a vast majority of organisms. Tyrosine and its hydroxylated product, dopa, form the starting material for melanin biosynthesis. Earlier studies by Raper and Mason resulted in the identification of dopachrome and dihydroxyindoles as important intermediates and paved way for the establishment of well-known Raper-Mason pathway for the biogenesis of brown to black eumelanins. Tyrosinase catalyzes the oxidation of tyrosine as well as dopa to dopaquinone. Dopaquinone thus formed, undergoes intramolecular cyclization to form leucochrome, which is further oxidized to dopachrome. Dopachrome is either converted into 5,6-dihydroxyindole by decarboxylative aromatization or isomerized into 5,6-dihydroxyindole-2-carboxylic acid. Oxidative polymerization of these two dihydroxyindoles eventually produces eumelanin pigments via melanochrome. While the role of quinones in the biosynthetic pathway is very well acknowledged, that of isomeric quinone methides, however, remained marginalized. This review article summarizes the key role of quinone methides during the oxidative transformation of a vast array of catecholamine derivatives and brings out the importance of these transient reactive species during the melanogenic process. In addition, possible reactions of quinone methides at various stages of melanogenesis are discussed.

The influence of esterification of carboxyl groups of ribonuclease-A on its structure and immunological activity

The effect of modification of carboxyl groups of Ribonuclease-Aa on the enzymatic activity and the antigenic structure of the protein has been studied. Modification of four of the eleven free carboxyl groups of the protein by esterification in anhydrous methanol/0.1 M hydrochloric acid resulted in nearly 80% loss in enzymatic activity but had very little influence on the antigenic structure of the protein. Further increases in the modification of the carboxyl groups caused a progressive loss in immunological activity, and the fully methylated RNase A exhibited nearly 30% immunological activity. Concomitant with this change in the antigenic structure of the protein, the ability of the molecule to complement with RNase-S-protein increased, clearly indicating the unfolding of the peptide "tail" from the residues for orthobenzoquinone reaction and the loss in immunological activity of the more etion of these derivatives as compared with the compact native conformation. The fact that even the fully methylated RNase-A retains nearly 30% of its immunological activity suggested that the modified protein contained antibody recognizable residual native structure, which presumably accommodates some antigenic determinants.

Chemical and physical modification of proteins by the hydroxide ion

Proteins are exposed to alkaline conditions during solubilization and/or purification, during food storage and processing, in removal of toxic constituents, and for characterization. During alkali treatment, there are changes in solubility and aggregation, hydrolysis, elimination reactions involving the side chains of certain amino acids, racemization of amino acid residues, addition of compounds to proteins, fragmentation of the peptide chain, as well as modification or elimination of nonprotein constituents. The rates of these reactions are affected by pH, temperature, cations (in some cases), ionic strength (in some cases), protein concentration, and to some extent by the specific nature of the protein. The general mechanisms and stoichiometry of these reactions are described. Other constituents of high protein foods also undergo reactions in alkaline solutions and the products of these reactions may in turn react with proteins. We have described the effect of alkali on enediol formation and fragmentation of carbohydrates, the hydrolysis of lipids in alkaline solution and effect on rate of peroxidation of the polyunsaturated fatty acids, the oxidation of amino acid residues, especially methionine, the oxidation of phenols to benzoquinones, and the catalytic effect of metal ions in alkaline solutions. Alkali treatment is also used in the specific modification of proteins to distinguish between O-glycosyl and amide-linked glycosyl groups, to effect specific cleavage of peptide bonds via beta elimination, in the formation of anhydrotrypsin, anhydrochymotrypsin, anhydrosubtilisin and thiol-subtilisin, and in formation of intrachain crosslinking in proteins.

Protein-polyphenol reactions. 1. Nutritional and metabolic consequences of the reaction between oxidized caffeic acid and the lysine residues of casein

1. Studies were made on the lysine content of casein reacted with caffeic acid oxidized aerobically under alkaline conditions of enzymically with tyrosinase (EC 1. 14. 18. 1). 2. Loss of fluorodinitrobenzene (FDNB)-reactive lysine was rapid at pH 10 and increased with time and the temperature of the reaction, with concentration of caffeic acid and with the oxygenation of the mixture. In presence of the enzyme mushroom tyrosinase, maximum reduction of reactive lysine occurred at pH 7 and was dependent on the reaction time and on the concentration of caffeic acid. 3. Reaction of alpha-formyl-LO-[U-14C]lysine with caffeic acid at pH 10 showed the rapid formation of five reaction products which appeared to polymerize gradually as the reaction progressed. 4. The nutritionally available lysine content of the casein-caffeic acid mixtures, as assayed with rats, was reduced after both alkaline and enzyme reactions, as were faecal digestibility, net protein ratio and net protein utilization. Biological value however was not reduced. 5. In metabolic studies using goat milk casein labelled with L-[3H]lysine and reacted with caffeic acid in the same way, the lysine-caffeoquinone reaction products were not absorbed by the rat but were excreted directly in the faeces. 6. The importance of the reaction of proteins with caffeoquinone and chlorogenoquinone (formed by the oxidation of caffeic and chlorogenic acids respectively) is discussed in relation to the production of sunflower protein, leaf protein and other vegetable-protein concentrates.

o-Benzoquinone-a reagent for determining conformational differences in related proteins

OBQ reacts specifically with methionine residues of RNAase A at highly acidic pH. At pH 3, whereas RNAase A does not react, RNAase S, S-protein and S-peptide all react with OBQ. RNAase A derivatives in which some of the methionines have been initially modified at pH 1, further react with OBQ at pH 3. Thus, reaction with OBQ can be used to monitor structural transitions and conformational differences involving methionines in proteins.

Chemical modification of methionines of ribonuclease A with o-benzoquinone

The accessibility of methionines in RNAase A to reaction with OBQ has been studied at highly acidic pH. The differences between the rate constants of reactions of the methionine and methionines of RNAase A with OBQ is a reflection on the limited accessibility of methionines in the protein conformation. Nevertheless, at sufficiently high OBQ concentration, all the four methionines of the enzyme can be modified. At lower concentration of OBQ, a derivative may be prepared in which a specific methionine is modified. The introduced chromophore ionizes at around pH 3 in this derivative. The derivative has partial activity towards RNA which is enhanced on addition of S-protein.

Proteolytic susceptibility and methionine modification of monodeamidated ribonuclease A

The susceptibility of a monodeamidated RNAaseA (RNAaseAa1) towards carboxypeptidaseA , alpha-chymotrypsin and pepsin has been studied. Similar to RNAaseA, the C-terminal of RNAaseAa1 is not available for carboxypeptidaseA hydrolysis. The thermal stability of RNAaseAa1 as probed through chymotryptic digestion is found to be less than that of RNAaseA. Preliminary chromatographic analysis of the digested material, however, suggests that the nature of thermal transition might be the same in the two proteins. Pepsin inactivates RNAaseAa1 more slowly than does RNAaseA. Accordingly, less peptide bonds, almost half that of RNAaseA, are cleaved by pepsin in RNAaseAa1. The accumulation of RNAase-P type intermediates is not evident during peptic digestion of RNAaseAa1. Reaction with O-benzoquinone at low pH shows that methionines of the deamidated protein seem to have higher reactivities. These observations indicate a different structure for RNAaseAa1 at elevated temperature and low pH.

Notes on a deamination method proposed for determining "chemically available lysine" of proteins

1. Details of a chemical method, using acetic acid and sodium nitrite, for determining ‘available lysine’ in plant materials are given. An analytical procedure was used which distinguished lysine from ornithine; the latter was liable to arise in the course of the deamination and hydrolysis procedure.

2. The experimentally determined ‘available lysine’ contents of various samples of leaf-protein concentrates showed significant positive correlations with previously reported values for their biological value and true digestibility. ‘Available lysine’correlated significantly with total lysine liberated by direct acid hydrolysis.