Periploca root oils as potent, diphenolase-selective tyrosinase inhibitors: chemotype profiling and non-competitive mechanism of 4-methoxy-2-hydroxybenzaldehyde.

Safer, plant-derived alternatives to hydroquinone are needed for managing hyperpigmentation. Steam-distilled oils of Periploca sepium (China) and Periploca angustifolia (Tunisia) were obtained in yields of 2.0 g kg-1 (w/w) and chemically profiled as phenolic chemotypes dominated by 4-methoxy-2-hydroxybenzaldehyde (788 and 746 g kg-1, respectively). The aldehyde was isolated at 970 g kg-1 purity with an overall yield of 1.5 g kg-1 for mechanistic studies. Both Periploca root oils selectively and near-completely inhibited mushroom-tyrosinase diphenolase at ≤ 50 μg mL-1 while preserving ≥ 75% monophenolase activity at the same doses. The Chinese oil was more potent (half-maximal inhibitory concentration (IC50) of 9.8 μg mL-1)) than the Tunisian oil (IC50 of 11.2 μg mL-1), consistent with its higher 4-methoxy-2-hydroxybenzaldehyde content and lower n-alkane fraction. The purified aldehyde (970 g kg-1) exhibited strong, non-competitive inhibition of diphenolase (IC50 of 9.6 μg mL-1; the Michaelis-Menten constant (Km) unchanged, maximum rate of reaction (Vmax) reduced) and was eight-fold more potent than arbutin under identical conditions. Structure-activity relationships indicated an optimal ortho-hydroxy/para-methoxy pharmacophore, whereas added hydroxy groups diminished potency. Acute toxicity data support a wide safety margin (oral median lethal dose (LD50) > 3000 mg kg-1 in rodents), and in silico ADMET predicted favorable dermal absorption, low cytochrome P450 inhibition liability, and cosmetic suitability. Periploca sepium chemotypes, standardized to ≥ 700 g kg-1 4-methoxy-2-hydroxybenzaldehyde and ≤ 50 g kg-1 total n-alkanes, are potent, diphenolase-selective tyrosinase inhibitors with promising safety attributes. These findings support development as clean-label actives for skin-whitening or food anti-browning, contingent on chemotype-directed quality control, confirmation on human tyrosinase/cellular models, and comprehensive chronic safety assessment.