Polyphenolase

C.79 Higher temperatures also accelerate a decline in polyphenolase activity, whereas peroxidase activity remains high. This favors thearubigen formation at the expense of theaflavin.92... 

The oxidation of catecholamines in the presence of tyrosinases and polyphenolases has been widely studied, and the subject has been adequately discussed in the literature. References 12 and 31-41 will serve as a guide to further reading on the subject. 

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-... 

To distinguish this type of activity from the one mentioned earlier, it is described as cresolase activity, whereas the other is referred to as catecholase activity. For both types of activity, the involvement of copper is essential. Copper has been found as a component of all polyphenolases. The activity of cresolase involves three steps, which can be represented by the following overall equation (Mason 1956) ... 

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. 

Oxidation of ornithine with consumption of atmospheric oxygen and release of ammonia is carried out by belladonna preparations (James and Beevers, 117a), and appears to be due to secondary oxidation by the quinones produced by the very active polyphenolase of belladonna tissues. 

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

STAFFORD, H.A., DRESLER, S., 4-hydroxycinnamic acid hydroxylase and polyphenolase activities in Sorghum vulgare. Plant Physiol., 1972,49, 151-157. 

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. 

The widespread occurrence of the polyphenolase in plants has, however, Ipng made it seem likely that these oxidases play an important part in respiration. Onslow s studies (1920-1924) emphasized the wide distribution in plant tissues of o-dihydric phenols such as caffeic and pro-tocatechuic acids, and focused attention on the possibility that the enzymatic oxidation of such substances was an important part of the plant respiratory process. Since then many investigators have attempted to show that polyphenolase acts as a terminal oxidase. 

Other evidence that polyphenolase functions as a terminal oxidase has been obtained in studies on tea (Sreerangachar, 1942 Roberts and Wood, 1951) and on spinach leaves (Bonner and Wildman, 1946), where it was found that 90 to 100 % of the respiration was inhibited by p-nitrophenol, an agent which inhibits polyphenolase but which is said to have no action on cytochrome. 

The idea that polyphenolase plays any significant role as a terminal oxidase in the potato has been criticized by Levy et al. (1948) on the grounds that the normal respiration of potato tissue is inhibited by CO and reversed by light—a characteristic of the iron oxidase systems, but not of... 

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. 

The most efficient of these phenolic compounds are those of the benzo-pyran class (quercitrin, ereodictyol) (Huszak, 1937). The action of peroxidase therefore resembles that of the polyphenolase in that the oxidation of ascorbic acid results from the intermediate formation of quinone derivatives. 

The conflicting evidence as to whether the cytochrome, the polyphenolase, or the ascorbic oxidase is the main terminal oxidase does not concern us here except that, as far as ascorbic acid is concerned, all or any of these... 

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