[Study on the interaction of cystine and polyphenol oxidase from nicotian tobaccum]

The interaction of cystine and polyphenol oxidase (PPO) from nicotian tobaccum has been studied. The results show that cystine has an inhibitory effect on the enzymatic activity, the mechanism of which might be that the thiolether in cystine coordinates with Cu2+ in the active site of PPO, and the inactivation constant value of PPO by cystine is 0.633 min(-1), while the substrates or the products inhibit their combination. Cystine can also be combined with the products of enzymatic reaction to form a colourless compound. Cystine can inhibit the enzymatic activity completely when the molar ratio of cystine to PPO approaches 16,000:1. The effect of cystine on the fluorescence changes with the molar ratio of cystine to the PPO. However, when the molar ratio gets to 75:1, cystine has no longer the effect on either the fluorescence spectra or the synchronous spectra. Microenvironment of trp residue in PPO is more hydrophobic than that of free trp in water.