Polyphenol browning reactions

Nutritional consequences of protein-polyphenol browning reactions. Xllth International Congress of Nutrition, San Diego, California, Abstract 260... 

The chemical and enzymatic browning reactions of plant polyphenols and their effects on amino acids and proteins are reviewed. A model system of casein and oxidizing caffeic acid has been studied in more detail. The effects of pH, time, caffeic acid level and the presence or not of tyrosinase on the decrease of FDNB-reactive lysine are described. The chemical loss of lysine, methionine and tryptophan and the change in the bioavailability of these amino acids to rats has been evaluated in two systems pH 7.0 with tyrosinase and pH 10.0 without tyrosinase. At pH 10.0, reactive lysine was more reduced. At pH 7.0 plus tyrosinase methionine was more extensively oxidized to its sulphoxide. Tryptophan was not chemically reduced under either condition. At pH 10.0 there was a decrease in the protein digestibility which was responsible for a corresponding reduction in tryptophan availability and partly responsible for lower methionine availability. Metabolic transit of casein labelled with tritiated lysine treated under the same conditions indicated that the lower lysine availability in rats was due to a lower digestibility of the lysine-caffeoquinone complexes. 

Enzymic browning reactions commonly occur when fruits and vegetables are bruised or peeled. They are due to the pol)rmeri-zation of phenolic compounds on exposure to air. The enzyme responsible for this browning is the polyphenol oxidase or pheno-lase enzyme which converts the phenol to its corresponding qui-... 

Phenolic compounds known as polyphenols (such as phenohc acids and flavonoids) are easily oxidised by atmospheric oxygen in reactions catalysed by oxidoreductases (o-diphenokO, oxidoreduc-tases). These reactions are known as enzymatic browning reactions (see Section 9.12). Autoxidation of plant phenols catalysed by heavy metal ions (mainly cupric and ferric ions) and oxidation by lipid hydroperoxides lead to similar products. The products of oxidation of the so-called o-diphenok (1,2-dihydroxybenzenes) are o-quinones (1,2-benzoquinones), highly reactive compounds that react with proteins (amino acids) and other food components with the formation of dark-coloured polymeric products resistant to proteolysis. 

Enzymes from the class of oxidoreductases catalysing enzymatic browning reactions are most often known trivially as polyphenol oxidases. Two groups of enzymes can be identified ... 

Another option to reduce or inhibit the enzymatic browning reaction is the use of chemical reagents. Many substances capable of inhibiting polyphenol oxidases are known, but the mechanism of their action is often unknown. Complexing agents are able to bind copper ions, which the enzyme requires. Interaction with... 

Polyphenoloxidase (PPO, EC 1.14.18.1) is one of the most studied oxidative enzymes because it is involved in the biosynthesis of melanins in animals and in the browning of plants. The enzyme seems to be almost universally distributed in animals, plants, fungi, and bacteria (Sanchez-Ferrer and others 1995) and catalyzes two different reactions in which molecular oxygen is involved the o-hydroxylation of monophenols to o-diphenols (monophenolase activity) and the subsequent oxidation of 0-diphenols to o-quinones (diphenolase activity). Several studies have reported that this enzyme is involved in the degradation of natural phenols with complex structures, such as anthocyanins in strawberries and flavanols present in tea leaves. Several polyphenols... 

Benzoquinones can act as dicarbonyl components of the Strecker reaction, readily form imines in the cold, and can be involved in the production of melanoidins. Benzoquinones in plant systems are usually derived from polyphenols and constitute well known intermediates in enzymic browning. They are also involved in the formation of melanin in animals, including humans. 

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