Enzymes polyphenolase

The most elementary biosensors are fruit pulps or slices which have been combined with amperometric electrodes. A well-known example is the ba-nanatrode (Wang and tin 1988). This sensor, most useful for demonstration experiments, contains a paste mix of banana pulp, nujol and carbon powder which has been pressed into a glass tube with an electric contact (Fig. 7.39). The mass contains the enzyme polyphenolase, which catalyses the oxidation of polyphenols, among them important biological messengers like dopamine. The sensor can be tested by means of simple compounds like catechol, which can be detected in beer. As a result of air oxidation, o-quinone is formed. The latter is an electrochemicaUy active compound which can be detected e.g. by differential-pulse voltammetry. 

Copper is present in foods as part of several copper-containing enzymes, including the polyphenolases. Copper is a very powerful prooxidant and catalyzes the oxidation of unsaturated fats and oils as well as ascorbic acid. The normal daily diet contains from 2 to 5 mg of copper, more than ample to cover the daily requirement of 0.6 to 2 mg. 

The enzymes involved in enzymic browning are known by the name polyphenoloxi-dase and are also called polyphenolase or phenolase. It is generally agreed (Mathew... 

The action of polyphenolases is detrimental when it leads to browning in bruised and broken plant tissue but is beneficial in the processing of tea and coffee. The enzyme occurs in almost all plants, but relatively high levels are found in potatoes, mush-... 

The substrates of the polyphenol oxidase enzymes are phenolic compounds present in plant tissues, mainly flavonoids. These include catechins, anthocyanidins, leucoantho-cyanidins, flavonols, and cinnamic acid derivatives. Polyphenol oxidases from different sources show distinct differences in their activity for different substrates. Some specific examples of polyphenolase substrates are chlorogenic acid, caffeic acid, dicatechol, protocatechuic acid, tyrosine, catechol, di-hydroxyphenylalanine, pyrogallol, and catechins. 

Polyphenol oxidases (e.g., tyrosinase, polyphenolase, phenolase, catechol oxidase, cresolase, catecholase) Chitinolytic enzymes... 

Laccase resembles the other polyphenolases in its action on ascorbic acid. Crude preparations oxidize ascorbic acid (Keilin and Mann, 1939), but their activity decreases as the enzyme is purified it can be restored by the addition dihydric phenols. The enzyme is without action on monohydric phenols, and consequently these do not induce the oxidation of ascorbic acid. Bertrand (1945a,b) has claimed that purified laccase, like ascorbic oxidase, will oxidize ascorbic acid directly, although no confirmation of this finding has so far been reported. 

Still more recent work by the same authors has suggested an alternative possibility. It has been generally assumed that L-ascorbic acid has no effect on the polyphenolase system other than its effect as a reducing agent for the o-quinone formed by the oxidation of the phenols. It has now been shown that ascorbic acid itself has an inhibitory action on the polyphenolase enzyme. When polyphenolase prepared from potato was treated with ascorbic acid under anaerobic conditions, and the ascorbic acid subsequently removed by dialysis, the activity of the enzyme was very considerably reduced. The enzyme after such treatment could not be reactivated by the addition of cupric salts and appeared to bo irreversibly inactivated. It was also shown that neither dehydroascorbic acid nor the further oxidation products of dehydroascorbic acid were responsible for this result. There is at present no explanation of the mechanism of this inhibitory action of ascorbic acid, but it is quite clear that, if these results are confirmed, other explanations are possible of why these enzymes do not exert their full potential effect in vivo. 

One reservation must, however, be made, that is, as to whether the particular member is competent to act as a substrate for the polyphenol-oxidases present. We have already (p. 268) seen that the 3-glycosides of quercetin and myricetin are not attacked by the tea polyphenolase system, whereas the aglycones themselves are readily attacked. It may prove to be important, in any tissue, whether the specific glycosides present are attacked by the enzyme present, or introduced from other sources, as in products of mixed origin. Such considerations as these undoubtedly operate in the manufacture of such products as cider and perry. 

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