Potent Tyrosinase Inhibitory Activity of Curcuminoid Analogues and Inhibition Kinetics Studies

Natural tyrosinase inhibitors from herbal plants are promising therapeutic agents for skincare and cosmetic products. Natural curcuminoids exhibit weak antityrosinase properties. The structural modification of curcumin, the major curcuminoid from Curcuma longa, gave 14 analogues. The tyrosinase inhibitory activity of the natural curcuminoids and the modified analogues on both L-tyrosine and DOPA substrates were evaluated. The inhibition kinetics were also undertaken. For analogues with potent activity on the L-tyrosine substrate, the isoxazole analogue 12 and two reduced analogues, hexahydrocurcumin (16) and the α,β-unsaturated analogue 17, showed IC50 values of 8.3, 14.6 and 9.4 µM, and were 20.9-, 11.9- and 18.4-fold more active, respectively, than kojic acid, the reference compound. For the analogues with potent antityrosinase on DOPA substrate, the dimethylated analogue 5 exhibited the strongest antityrosinase activity against the DOPA substrate, with the IC50 value of 8.0 µM, which was 16.6-fold more active than kojic acid. The inhibition kinetics revealed that curcuminoid 5 could bind with both free enzyme and with the enzyme–substrate complex. It acted as a competitive–uncompetitive mixed-II type inhibitor. Curcuminoid 17 could bind with both free enzyme and the enzyme–substrate complex. The results indicated that 17 acted as a competitive–uncompetitive mixed-I type inhibitor, while curcuminoid 12 was a noncompetitive inhibitor which bound with both free enzymes and the enzyme–substrate complex. These potent analogues might serve as new potential tyrosinase inhibitors for the prevention and treatment of skin pigmentation disorders.

Effect of Extracellular Tyrosinase on the Expression Level of P450, Fpr, and Fdx and Ortho-hydroxylation of Daidzein in Streptomyces avermitilis

We have reported that the expression of CYP105D7 in Streptomyces avermitilis produces 112.5 mg L^-1 of 7,3',4'-trihydroxyisoflavone (3'ODI) in 15 h of the reaction time, when 7,4'-dihydroxyisoflavone (daidzein) is used as a substrate. Although production is significant, rapid degradation of 3'ODI after 15 h was observed in a whole-cell biotransformation system, suggesting the further modification of 3'ODI by endogenous enzymes. In this present study, the effect of deletion of extracellular tyrosinase (melC2) in S. avermitilis for 3'ODI production as well as the expressions of CYP105D7, ferredoxin (Fdx), and ferredoxin reductase (Fpr) were investigated. The result revealed that daidzein hydroxylation activity in the ∆melC2 mutant decreased by 40% compared with wild-type S. avermitilis. Further, melC2 deletion significantly affects the messenger RNA (mRNA) expression profile of CYP105D7 and its electron transfer counterparts. Real-time PCR analysis of 9 Fdx, 6 Fpr, and CYP105D7 revealed a significant decrease in mRNA expression level compared to wild-type S. avermitilis. The result clearly shows that the decrease in daidzein hydroxylation activity is due to the lower expression level of CYP105D7 and its electron transfer counterpart in the ∆melC2 mutant. Furthermore, melC2 deletion prevents the degradation of 3'ODI.

Moracin VN, A New Tyrosinase and Xanthine Oxidase Inhibitor from the Woods of Artocarpus heterophyllus

Bioactivity-guided fractionation of the woods extract of Artocarpus heterophyllus collected in Vietnam revealed that a new 2-arylbenzofuran, moracin VN (1) together with two known compounds were isolated. Compound 1 possessed the inhibitory activity on tyrosinase with IC50 value of 0.82 μM, more potent than the positive control kojic acid (IC50, 44.6 μM). Compound 1 also showed moderate inhibitory activity on xanthine oxidase with IC50 value of 22.8 μM. The kinetic study of tyrosinase was performed on moracin VN (1) showed non-competitive inhibition with Ki value of 2.40 μM.