Abstract:
Polyphenol oxidase (PPO) enzyme is a metalloenzyme containing a copper and catalyzes the conversion of phenolic compounds to quinones and assists the polymerization of their products. It causes browning reactions in fruit and vegetables due to oxidation during handling and storage processes. This enzymatic browning reaction is the one of the biggest problem in the food processing industries during processing and storage of vegetables and fruits. Lately, researchers are very interested in developing new PPO inhibitors to slow the enzymatic browning. Natural anti-brown compounds commonly used for the PPO enzyme are honey, aliphatic alcohols, ascorbic acid and cysteine. In addition to these substances, some metals can affect enzyme activities positively or negatively. For example, iron, copper and zinc are essential metals for plant life, but some heavy metals such as mercury, lead, etc. can adversely affect enzymes in the plant defense system. In this work, the effects of some metals, anti-browning compounds and their complexes were examined on the PPO enzyme from princess tree (Paulownia tomentosa) leaf. The results demonstrated that glutathione (GSH) was more powerful anti-browning compound than the others. Ni(II) and Mn(II) increased princess tree PPO activity. But, Pb(H) was the efficient PPO inhibitor. Na(I), Cu(II) and Fe(II) had no considerable effect on the PPO enzyme activity. Additionally, the effects of metal and anti-browning compound mixtures on the PPO enzyme were examined. Our results showed that metal-L-Cys, metal-Ascorbic acid and metal-GSH complexes had inhibitory effects but metal- EDTA complex did not have a significant effect on princess tree PPO enzyme activity.
