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-alpha-methyl-Dopa, DL-alpha-methyl-Dopa; L-isoproterenol and DL-isoproterenol. V-max(D) 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 delta(3) and delta(4) values. The effect of the nucleophilic power of each o-diphenol and the different sizes of the side substituents could explain the different V-max(D) values observed for different o-diphenols. However, K-m(D) 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.