Shahriar Saeidian
L-glycine and L-cysteine exhibit, strong inhibition of partial purified PPO of wild pear. The concentration of Lglycine inhibiting PPO activity by 50% (IC50) was 0.8 and 0.5 mM for catechol oxidation and 0.75 and 0.5 mM for pyrogallol oxidation at pH 5 and 7, respectively. IC50 for L-cysteine calculated, 0.6 and 0.35 for catechol oxidation and 0.55 and 0.75 mM for pyrogallol oxidation at pH 5 and 7 respectively. The inhibition of partial purified PPO activity is pH and inhibitor dependent. Kinetic studies indicate that L-glycine is a competitive and noncompetitive inhibitor and L-cysteine is competitive and noncompetitive inhibitor of partial purified PPO at pH 5 and 7 in presence of catechol. These inhibitors showed different type of inhibition at pH 5 and 7 in presence of pyrogallol, too. Vmax and Km for pyrogallol oxidation at pH 5 and in presence of L-glycine (1M) was 320 unit/mg protein and 6.7 mM. Vmax for pyrogallol oxidation at pH 7 and in presence of L-glycine was 180 unit/mg protein, with a Km of 5.2 mM. Kinetics parameters indicated the highest catalytic efficiency ( units mg–1 prot mM–1) with pyrogallol and inhibitors of L-glycine and L-cysteine at pH 5 (47.7 and 34.4) , and minimum of catalytic efficiency was earned for inhibitors at pH 7 for catechol oxidation. These date showed that Isoform of PPO at pH 7 has higher sensitivity related to another isoform of PPO at pH 5.
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