South Korean researchers from Dongshin University and B & Tech Co. found that the postbiotic metabolite, phenyllactic acid (PLA), significantly inhibited melanin production in melanoma cells.
The effects were achieved without any observed toxicity. At a high concentration, PLA proved even more effective than the established cosmetic standard, arbutin.
The scientists detailed the discovery in a paper published in Cosmetics, stating that PLA acted through a unique dual mechanism, competitively inhibiting tyrosinase while also regulating the expression of related enzymes.
The study demonstrated that PLA effectively suppressed melanogenesis through both enzymatic and transcriptional mechanisms. These results suggested that PLA, with its low toxicity and probiotic origin, is a promising candidate for development as a safe and effective skin lightening ingredient in functional cosmetics.
Postbiotics as stable, safer alternatives
The cosmetics industry is increasingly turning its attention to postbiotics — metabolites produced by beneficial microorganisms like probiotics — as a source of novel active ingredients. Unlike live bacteria, postbiotics offer enhanced safety and stability, making them easier to incorporate into formulations.
Currently used whitening agents like arbutin and kojic acid are often limited by concerns over cytotoxicity and instability during prolonged use. This underscored a critical need for alternative agents that could offer both high efficacy and improved safety.
The research team targeted PLA because it is a key metabolite of L. reuteri and has a structure that chemically resembles the natural substrates of tyrosinase, the enzyme responsible for kicking off melanin production.
Dual-action mechanism
The researchers first confirmed the presence of PLA as a major metabolite in the L. reuteri culture broth using high-performance liquid chromatography (HPLC). Next, they used molecular docking simulations to predict how PLA would interact with tyrosinase.
The results suggested that PLA could physically bind within the enzyme’s catalytic pocket, supporting its potential as a competitive inhibitor.
To validate this in the lab, they tested PLA’s effect on mushroom tyrosinase, a common model for the human enzyme. The results showed that PLA inhibited the enzyme’s activity in a concentration-dependent manner.
Stronger affinity for second step of pigmentation
Detailed enzyme kinetic analysis revealed that PLA acted as a competitive inhibitor. This means PLA physically competed with the natural substrate — L-tyrosine or L-DOPA — for a spot in the enzyme’s active site.
The researchers also found that PLA had a much stronger inhibitory effect on the second step of the melanogenesis pathway, known as diphenolase activity, which involves the oxidation of L-DOPA.
The inhibition constant for the L-DOPA reaction was markedly lower than for the L-tyrosine reaction. This indicated that PLA bound more strongly to the active site during the second stage of the process, indicating a preferential inhibition of the diphenolase activity.
The researchers speculated that the structural features of PLA, particularly its phenyl ring, mimicked the aromatic structure of L-DOPA to enable this stronger, competitive binding.
Superior cellular performance
To see how well PLA might work in a real-world product, the scientists tested it on special skin cells called B16F10 melanoma cells, which are often used to study skin colour.
First, they checked to make sure PLA was safe using an MTT assay, and confirmed that PLA was not harmful or poisonous to the cells at the amounts they tested. In fact, more than 90% of the cells remained healthy and alive.
Next, they encouraged the cells to produce melanin and found that PLA greatly reduced the amount of melanin produced as they increased the dose. At the highest dose (5mM), PLA reduced melanin production by about 48% compared to the untreated, stimulated cells.
The scientists pointed out that this whitening effect was stronger than arbutin.
Unique regulation of melanogenesis-related enzymes
In addition, the study found that PLA works in a unique way inside the cells. Melanin production is controlled by a set of key proteins, where MITF tells the cell to start making pigment enzymes and TRP-1 and TRP-2 are the necessary tools (enzymes) that carry out the actual work of making melanin.
Most existing skin-whitening ingredients work by lowering the MITF level, which then causes the downstream tools (TRP-1 and TRP-2) to also be lowered. However, the analysis showed that PLA increased the amount of MITF, and at the same time, TRP-1 and TRP-2 were strongly reduced.
The researchers suggested that PLA was not turning off the initial instructions. Instead, PLA might have broken or blocked the MITF after it was already made. This stopped the main instructions from being delivered correctly, which effectively shut down the later steps of the melanin production line (TRP-1 and TRP-2).
This dual action sets PLA apart from traditional whitening agents, which usually only focus on either directly blocking the main enzyme or simply reducing the initial instructions.
Looking ahead for formulation
The findings lay a strong scientific foundation for developing PLA as a new, high-performance skin lightening cosmetic ingredient. Beyond its efficacy and safety, PLA’s chemical properties make it an attractive candidate for formulators.
The researchers concluded: “As an aromatic organic acid with high water solubility and postbiotic properties, PLA is expected to remain stable in mildly acidic environments (pH 3 to 5). Therefore, it may be suitable for use in aqueous cosmetic formulations such as toners or essences for topical application, although further studies are needed to optimise formulation stability.”
Source: Cosmetics
“Skin Whitening Effect and Molecular Mechanism of Phenyllactic Acid (PLA) Derived from Limosilactobacillus reuteri Culture Broth”
https://doi.org/10.3390/cosmetics12060258
Authors: Kim Ki-Min, et al.
