Why proline added to the substrate?

We conducted a test of the polyphenol oxidase (PPO) activity in apples. We the inhibitors of various tests with it. But one question being asked is why is it added to the substrate proline. The substrate is catechol. Ive looked everywhere I can think of! Thanks in advance! Xxx

From what I can find on the Internet, it appears that proline using the enzyme tyrosinase is also called, because it reacts with the product (o-quinones) and produces a color. Therefore, catechol is oxidized by the PPO and the product is an o-quinone that is detected by reaction with proline to give color.

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