The effect of pH on horseradish peroxidase‐catalyzed oxidation of melatonin: production of N¹‐acetyl‐N²‐formyl‐5‐methoxykynuramine versus radical‐mediated degradation

Abstract

There is a growing body of evidence that melatonin and its oxidation product, N(1)-acetyl-N(2)-formyl-5-methoxykynuramine (AFMK), have anti-inflammatory properties. From a nutritional point of view, the discovery of melatonin in plant tissues emphasizes the importance of its relationship with plant peroxidases. Here we found that the pH of the reaction mixture has a profound influence in the reaction rate and products distribution when melatonin is oxidized by the plant enzyme horseradish peroxidase. At pH 5.5, 1 mm of melatonin was almost completely oxidized within 2 min, whereas only about 3% was consumed at pH 7.4. However, the relative yield of AFMK was higher in physiological pH. Radical-mediated oxidation products, including 2-hydroxymelatonin, a dimer of 2-hydroxymelatonin and O-demethylated dimer of melatonin account for the fast consumption of melatonin at pH 5.5. The higher production of AFMK at pH 7.4 was explained by the involvement of compound III of peroxidases as evidenced by spectral studies. On the other hand, the fast oxidative degradation at pH 5.5 was explained by the classic peroxidase cycle.

Enzymatic Degradation of A2E, a Retinal Pigment Epithelial Lipofuscin Bisretinoid

“…Importantly, as has been shown previously for other substrates, 20 HRP can act on A2E at pH 5.5, the pH of lysosomes wherein the bisretinoids of RPE reside. We have characterized the structures of four of the largest degradation products; continued decline with time in the peak intensities attributable to these products indicated that enzyme-mediated cleavage was progressive.…”

The co‐catalytic effect of chlorpromazine on peroxidase‐mediated oxidation of melatonin: enhanced production of N1‐acetyl‐N 2‐formyl‐5‐methoxykynuramine

“…3). Peak 1 correlates with hydroxymelatonin, which was previously identified by us and others [18–20]. Peak 2 correlates with AFMK, one of the main products of enzymatic and nonenzymatic oxidation of melatonin [18–25].…”

“…Recently, we demonstrated that the pH of the medium has a profound influence on the rate and products profile during the oxidation of melatonin [18]. For this reason, we studied how CPZ could affect the oxidation of melatonin at different pH.…”

Protection against cell death and sustained tyrosine hydroxylase phosphorylation in hydrogen peroxide‐ and MPP+‐treated human neuroblastoma cells with melatonin

“…In addition, it has been demonstrated that the metabolites of melatonin including N 1 ‐acetyl‐ N 2 ‐formyl‐5‐methoxykynuramine and N ‐acetyl‐5‐methoxykynuramine have effective antioxidants properties. It seems some of the antioxidative roles of melatonin could have been a consequence of its metabolites [23–25]. Jou et al.…”