Cyclodextrins as activator and inhibitor of latent banana pulp polyphenol oxidase

The effect of cyclodextrins (CDs) on o-diphenol oxidation catalyzed by bana na polyphenol oxidase (PPO) was studied. The oxidation of dopamine, the nat ural substrate of banana, in the presence of cyclodextrins was unaffected, because this hydrophilic phenol does not form inclusion complexes with CDs. However, when a hydrophobic phenol such as tert-butylcatechol (TBC) was us ed, a marked inhibition was observed with beta-, hydroxypropyl-beta-, and m altosyl-beta- CDs. This inhibition was due to the complexation of TBC in th e CD core, demonstrating that banana pulp PPO worked only toward free subst rate and not toward the complex TBC-CDs. In addition, the effect of some in hibitors in the presence of CDs and dopamine as substrate was studied. Incr easing concentrations of CDs, in the presence of two inhibitors (4-iodophen ol and cinnamic acid) were able to activate the inhibited enzyme to reach t he noninhibited level by complexing the inhibitors in the hydrophobic core of the CDs. This dual effect of CDs as activator and inhibitor was tested i n crude banana pulp extracts, with surprising activation effects never befo re described being observed.