Graduate Thesis Or Dissertation
 

Inhibition of polyphenol oxidase by sulfur dioxide

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  • 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.
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file details SOTOLUIS1983.pdf 2017-08-15 Público Baixar