CYSTEINE AS AN INHIBITOR OF ENZYMATIC BROWNING .2. KINETIC-STUDIES

The effect of cysteine and cysteine-quinone addition compound (CQAC) on apple polyphenol oxidase (PPO) activity was investigated by HPLC and polarography, using 4-methylcatechol (4MC), chlorogenic acid (CG), and (-)-epicatechin (EC) as substrate. With 2.2 mM 4MC, in the presence of cysteine at thiol to phenol ratios above 1, there was no effect in the degradation rates of 4MC, while CQAC formation was proportional to the 4MC loss but without color formation. For cysteine to phenol ratios below 1, the degradation rate of 4MC was slowed down; the lower the cysteine content, the earlier the decrease in 4MC degradation was observed. In the latter case, the CQAC content increased and then decreased but developed a strong color. Similar results were obtained with CG or EC as substrate. In the presence of CQAC, polarography experiments showed that they were not substrate but competitive inhibitors of apple PPO with a slightly higher affinity than their phenol precursors. HPLC analysis showed that Cys-5-MC (the CQAC derived from 4MC) was rapidly degraded, whereas 4MC degradation was slowed with a strong violet color formation. Nonenzymatic cooxidation reactions between CQAC and o-quinone leading to the regeneration of phenol and the formation of pigments were observed. Cysteine at a higher concentration prevented color development by trapping o-quinones as colorless CQAC, while at low amounts the o-quinones formed in excess can cooxidize CQAC, leading to a phenol regeneration with a deep color formation.