Cysteine as an Inhibitor of Polyphenol Oxidase

Abstract

The effect of L-cysteine on mushroom polyphenol oxidase (PPO) activity was investigated using three spectrophotometric assays. The formation of pigment (melanin), o-phenylquinone and cysteine-quinone adduct from catechol were each assayed under similar conditions, Cysteine had two effects; Jirst, a lag phase was seen when melanin formation was measured, and secondly, the rate of browning was decreased after the lag phase. The lag phase was not observed when the formation of cysteine-quinone adduct rather than melanin formation was measured. This suggests that the lag phase observed in melanin formation is due to adduct formation and not quinone reduction. The velocity of adduct formation was similar to the velocity of o-phenylquinone and melanin formation without cysteine suggesting that reduction of o-phenylquinone to catechol was not sign$cant. The structure of the adduct formed between cysteine and ophenylquinone was synthesized and unequivocally determined by NMR spectroscopy to be S-(2,3-dihydroxyphenyl)cysteine.

Diphenol Oxidases, Enzyme-catalysed Browning and Plant Disease Resistance

“…Sulphur amino acids, such as cysteine, and the tripeptide glutathione (y-glutamyl-cysteinyl-glycine, the reduced form) have been reported as effective inhibitors of browning in fruit juices (Walker, 1964a;Walker and Reddish, 1964;Montgomery, 1983;Dudley and Hotchkiss, 1989;Molnar-Perl and Friedman, 1990a,b;Richard et al, 1991, Robert et al, 1996 and, considering its low cost and the fact that it is a primary biometabolite, it is surprising that its use in industry has not been more widespread. Iyengar and McEvily (1992) reported that the concentration required for acceptable inhibition of enzymic browning had negative effects on taste but this is dependent on the system to be inhibited.…”

“…However, sulphites are also known to be mildly corrosive and SO2 is an effective bleaching agent so its use in food processing is restricted by regulatory limits. Even within these limits its use in foods is a topic of current media attention, more especially since some sulphite-sensitive individuals (usually asthmatics) have died due to large doses ingested from treated foods (Dudley and Hotchkiss, 1989). The use and labelling of sulphiting agents in foods has been restricted by new US Food and Drug Administration (USFDA) regulations and in 1986 a ban on the use of sulphites on fresh fruits and vegetables, including in restaurant salad bars, was imposed.…”

Quality changes in fresh-cut pear slices as affected by controlled atmospheres and chemical preservatives

“…Three mechanisms have been proposed to explain how thiol compounds inhibit enzymatic browning; (1) reduction of the o-quinone back to o-dihydroxyphenol (Kahn, 1985); (2) direct inhibition of PPO (Dudley and Hotchkiss, 1989;Robert et al, 1996); and (3) the formation of a colorless cys-quinone adduct (Richard et al, 1991). When cysteine is used as an inhibitor of enzymatic browning on sliced apples (Walker and Reddish, 1964) or pears (Sapers and Miller, 1998), pinkish-red colored compounds are formed due to phenol regeneration with deep color formation (Richard-Forget et al, 1992).…”

Dual effects of whey protein isolates on the inhibition of enzymatic browning and clarification of apple juice

“…Cys retards browning by reacting with o-quinones to form stable, colourless compounds. It has been reported that Cys forms a product with catechol, which lowers the PPO activity (Dudley & Hotchkiss 1989). According to the results obtained (Figure 2), Cys appeared to inhibit completely the PPO activity at the concentrations above 1.0 g/l.…”