Skin Whitening Effect and Molecular Mechanism of Phenyllactic Acid (PLA) Derived from Limosilactobacillus reuteri Culture Broth

Skin hyperpigmentation is primarily regulated by melanogenesis, in which tyrosinase and related enzymes play pivotal roles. Probiotics have recently been attracting attention as a cosmetic ingredient due to their skin-friendly and eco-friendly properties. In particular, microbial metabolites, known as postbiotics, are gaining attention for their superior safety, stability, and efficacy compared with probiotics. In this study, we investigated the whitening effect and molecular mechanisms of phenyllactic acid (PLA), a metabolite derived from Limosilactobacillus reuteri (L. reuteri) culture broth. In B16F10 melanoma cells, the effects of PLA were evaluated by measuring melanin content, cellular tyrosinase activity, enzyme kinetics, and the expression of melanogenesis-related proteins. PLA significantly inhibited melanin production and cellular tyrosinase activity in α-MSH–stimulated B16F10 melanoma cells without inducing cytotoxicity. PLA downregulated tyrosinase-related proteins such as TRP-1 and TRP-2, and competitively inhibited tyrosinase. The inhibition constants (Ki) for L-tyrosine and L-DOPA were 12.63 mM and 0.68 mM, respectively. These findings suggest that PLA, a postbiotic derived from lactic acid bacteria, may serve as a safe and effective whitening ingredient, providing a scientific basis for the development of functional skin-whitening cosmetics.