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Inhibition of polyphenol oxidase by sulfur dioxide

Inhibition of polyphenol oxidase (PPO) by sulfur dioxide (SO₂)
was studied using three different sources of PPO (banana, mushroom,
and pear). Several methods to detect PPO activity were tested due to
SO₂, interference in some of the assays. The method using
2-nitro-5-thiobenzoic acid (TNB) to react with the quinones was found
to be the most reliable for assaying PPO activity in the presence of
SO₂ whereas, the oxygen electrode and spectrophotometric methods
were not suitable. When PPO was exposed to SO₂ prior to the
substrate addition, it was inhibited irreversibly. Trials to
regenerate the PPO activity using extensive dialysis, column
chromatography, and addition of copper salts were not successful.
Experiments using Cu(II) and the TNB method to regenerate the activity
of the pear PPO apoenzyme that was not exposed to SO₂, also showed
the turnover between Cu(I) and Cu(II) during the enzyme oxidation of o
-phenols. Increased concentrations of SO₂ and pH less than 5
enhanced the inhibition of PPO by SO₂. At pH 4 concentrations
greater than 20 ppm completely inhibited 1,000 units of PPO activity
almost instantaneously. This suggests SO₂ as such as the main form inhibiting PPO. Kinetic studies confirmed the irreversibility of the
inhibition. Purified pear PPO was used to determine binding of
³⁵SO₂ to the enzyme. Column chromatography, extensive dialysis
and gel electrophoresis did not show SO₂ bound to PPO protein.
Formation of extra bands on gel electrophoresis in the SO₂
inhibited pear PPO fractions was demonstrated. This and other evidence
suggests that there was a modification of the protein structure of
PPO, with retention of protein integrity. Also it is suggested that
this was the main mode of direct irreversible inhibition of PPO by
SO₂. / Graduation date: 1984
Date03 October 1983
CreatorsSayavedra-Soto, Luis Alberto
ContributorsMontgomery, Morris W.
Source SetsOregon State University
Detected LanguageEnglish

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