Leptospermone acts as a predominantly non-competitive β-triketone inhibitor of tyrosinase

Leptospermone, a β-triketone derived from Leptospermum scoparium seed oil, was evaluated as a predominantly non-competitive inhibitor of tyrosinase. The oil inhibited mushroom tyrosinase at 100 µg/mL and retained strong activity at 50 µg/mL (L-tyrosine 75.2%, L-DOPA 71.4%). GC/MS profiling identified ten volatiles spanning six chemical classes, with leptospermone (23.8%) as the active. Guided fractionation confirmed leptospermone as the inhibitor of both substrates with comparable intensity (IC50, 30.3–31.5 µg/mL), whereas most co-constituents were inactive at ≤ 100 µg/mL; naphthalene showed moderate effects. Across an analogue panel, increasing methylation on the 1,3,5-trione ring enhanced potency, with 2,2,4,4,6,6-hexamethyl-1,3,5-trione most active against L-DOPA (IC50, 23.1 µg/mL). Kinetic analyses demonstrated decreases in Vmax with unchanged Km for leptospermone and active β-triketones on both substrates, indicating predominantly non-competitive or mixed-type inhibition consistent with a mechanism that may involve allosteric or secondary-site interactions; in contrast, arbutin was competitive on monophenolase and weaker on L-DOPA. This substrate-independent, allosteric profile aligns with SAR and a structural rationale in which a carbonyl-rich core provides H-bond acceptor capacity and ring methylation increases hydrophobic surface area, stabilizing recognition at a secondary pocket rather than the dicopper active site. Translation will require verification on human tyrosinase isoforms and adherence to toxicological guidelines, because mammalian safety data for leptospermone are limited and highly methylated analogues exhibited cytotoxicity above their enzymatic IC50 in our set. These results support β-triketones as substrate-independent tyrosinase inhibitors with optimization space for balanced efficacy and safety in food and cosmetic applications. Validation in product-relevant matrices and stability testing are further priorities.