Study of Stereospecificity in Pear and Strawberry Polyphenol Oxidases

The stereospecificity in pear and strawberry polyphenol oxidases (PPO) is studied in this paper. For this purpose several enantiomorphs of o -diphenols were assayed: l -Dopa, dl -Dopa, d -Dopa; l -α-methyl-Dopa, dl -α-methyl-Dopa; l -isoproterenol and dl -isoproterenol. values obtained for the stereoisomers within each series of o -diphenol were the same. This lack of stereospecificity in the transformation steps of the catalytic mechanism could be explained on the basis of the nucleophilic power of each o -diphenol, which was related to δ 3 and δ 4 values. The effect of the nucleophilic power of each o -diphenol and the different sizes of the side substituents could explain the different values observed for different o -diphenols. However, values were lower for l -isomers than those for d -isomers, which revealed the existence of stereospecificity in the affinity of PPO toward its substrates. The kinetic behaviors were interpreted in the light of a previously proposed reaction mechanism for the diphenolase activity of PPO from different fruits and vegetables.