INHIBITION OF POLYPHENOL OXIDASE BY THIOLS IN THE ABSENCE AND PRESENCE OF POTATO TISSUE SUSPENSIONS

To contribute to the development of sulfite alternatives to prevent enzymatic browning in fruits and vegetables, the inhibition of polyphenol oxidase (PPO) by structurally different sulfhydryl compounds (thiols) and by sodium bisulfite in the absence and presence of suspensions of dehydrated potatoes was measured. Relative potencies (I-50 values), defined as the millimolar concentration of the inhibitor needed to reduce 400 units of PPO activity by 50%, were calculated from regression equations of the concentration dependence of the inhibition at 25 degrees C. For solutions of pure PPO, the 150 values ranged as follows: N-(2-mercaptopropionyl)glycine, 0.058 (the most potent inhibitor); L-cysteine methyl ester, 0.099; 2-mercaptopyridine, 0.100; L-cysteine ethyl ester and reduced glutathione, 0.12; 2-mercBptoethanesulfonic acid, 0.14; sodium bisulfite, 0.21; homocysteine, 0.23; N-acetyl-L-cysteine, 0.27; L-cysteine, 0.35; and cysteinylglycine, 0.43. Studies on the influence of temperature with six inhibitors, each tested at the I-50 concentration listed above, in the range 25-55 degrees C showed that although heat alone partly inactivated the enzyme, the presence of thiols such as cysteine, N-acetyl-L-cysteine, and reduced glutathione markedly increased the inactivation. However, this was not the case for cysteine ethyl and methyl esters. Inhibition studies with NDA 1725 and Russet potato suspensions at 25 degrees C revealed that the inhibitors were less effective in the heterogenous food milieu and that the relative effectiveness of structurally different thiols differed from those observed in PPO solutions. Mechanisms are described to explain the inhibition of PPO and the prevention of enzymatic browning by SH-containing compounds. Possible approaches to applying the results to the development of useful browning inhibitors for foods are also discussed.