Screening of ninety herbal products of commercial interest as potential ingredients for phytocosmetics

Abstract Elastase and tyrosinase are important targets both for cosmetics and for dermatological disorders. In this work, ninety herbal products were tested as inhibitors of these two enzymes. Eleven extracts resulted strongly active. Four out of them (Camellia sinensis, Ginkgo biloba, Rhodiola rosea, Vitis vinifera) inhibited both enzymes, five (Glycyrrhiza glabra, Ribes nigrum, Rheum officinale, Salvia officinalis, Tilia platyphyllos) were active against tyrosinase only, and two (Ceterach officinarum and Cinnamomum zeylanicum) proved selectively active against elastase. The IC50 ranged from 3.1 to 104.9 μg/mL and 19.3 to 164.3 μg/mL, against elastase and tyrosinase, respectively. The most active extracts resulted enriched in flavonoids (from 1.47 to 56.47 mg RE/g of extract) and phenolics (from 37.43 to 123.56 mg GAE/g of extract), indicating also an antioxidant potential. Finally, a positive correlation between enzymatic bioactivities and phenolic content was also established.


Introduction
The demand for new skincare ingredients, especially based on natural products, strongly increased during the last few decades 1,2 . Plant bioactive metabolites are most of the time free from harmful side effects, hence, great importance is given to the research of naturally occurring anti-ageing agents. Several plants proved to be effective to slow down skin ageing 3 , acting as antioxidants, protecting skin against solar radiations 4 , and/or modulating the activity of enzymes involved in the ageing processes 5 , among which elastase (Ela) and tyrosinase (Tyr) are of remarkable importance.
Elastase activity increases significantly with age and after chronic UV-B irradiation 6 , resulting in sagging due to loss of skin elasticity, thus inhibition of this enzyme is valuable strategy to slow down the intrinsic and extrinsic ageing processes 7 . Tyrosinase is responsible for skin hyper-pigmentation, as in case of melasma, freckles, ephelide and senile lentingines 8 being an important target for skin-whitening agents.
This work aimed at identifying herbal products endowed with Tyr and/or Ela inhibitory activity (IA), thus potential anti-ageing agents. The bioactivity screening was carried out on extracts obtained from ninety commercial plants, widely used as ingredients for herbal preparations, including botanicals, herbal teas, and food supplements 9,10 . Considering the importance of polyphenols as antioxidant, the total phenolic and flavonoid content of the extracts was also determined and their content was also statistically correlated to the percentages of enzymatic inhibition.

Plant material and extracts preparation
Plant samples were kindly supplied by Biokyma S.r.l., Anghiari (AR) Italy, and identified by Dr. Franco Maria Bini and vouchers of crude drugs were deposited in Department of Pharmacy and Biotechnology, University of Bologna (via Irnerio 42, Bologna, Italy) and reported in Table 1.
Thirty mg of dried and powdered plant material were extracted by sonication for 30 min using 1.5 mL of EtOH/H 2 O (1:1). Crude extracts were obtained as reported by Chiocchio et al. 11 .

Enzyme inhibitory assays, total phenolic and flavonoid content
The assays were performed according to the methods described by Chiocchio et al. 11 , with slight modification for elastase inhibitory assay, where N-succinyl-Ala-Ala-Pro-Phe was used as substrate and p-phenylmethylsulfonyl fluoride (PMSF) from 1 to 250 lg/mL was used as positive control. For PMSF the assay was performed in 5% DMSO, thus, a proper negative control in the same conditions was used for the IC 50 calculation.
The kinetic parameters for the enzymatic reactions in the assay conditions were K M ¼ 0.2 mM for both enzymes and V max ¼ 6 lmol/min for elastase and V max ¼ 10 lmol/min for tyrosinase.
The assays for phenolic and flavonoid contents were performed in Spectrophotometer (Jasco V-530) as described by Chiocchio et al. 11 .

Statistical analysis
Values were expressed as the mean ± SD of three independent experiments (each one performed in duplicate). Statistical analyses were performed using R Studio software (version 1.1.463) based on the R software version 3.5.2 12 .
Samples were compared by one-way analysis of variance (ANOVA) performed with "aov" function using "stats" package, Table 1. Plants tested in this study, including their botanical name, family, organ/s used, voucher number, percentage of tyrosinase (Tyr IA %). and elastase inhibitory (Ela IA %) activity at 50 lg/mL, total phenolics (TPC) and flavonoids (TFC) content expressed in mg of gallic acid (GA) equivalent/mg of extract and mg of rutin (R) equivalent/mg of extract respectively. followed by Tukey's Honestly Difference (HSD) post-hoc test using TukeyHSD function presents in "stats" package 13 , considering significant difference at p values < 0.05. In order to determine the correlation between total phenolic and flavonoid content and enzymatic activities, Pearson correlation coefficient (r) was evaluated with "cor.test" function using "stats" package 14 .

Results and discussion
A first bioactivity screening was carried out on extracts at the fixed concentration of 50 lg/mL. The obtained results (Table 1) allowed the selection of eleven extracts, whose IA was higher than 30%, namely: The IC 50 values of Ela IA of the six selected samples ranged from 3.1 ± 1.9 to 104.9 ± 2.1 lg/mL (Figure 1(A)), and among them, RRO and COF resulted the most potent elastase inhibitors (EI). These results are particularly promising, considering that the positive control (PMSF) used for Ela inhibitory assay showed IC 50 of 42 lg/mL (241 lM). The extract of COF resulted the most potent Ela inhibitor of this screening, with an IC 50 value of 26 ± 0.3 mg/mL. Regarding VVI, this study reported for the first time its activity against Ela, while its Tyr IA was already known 15 . Finally, this study provides new bioactivity data also for CZE, which is generally used as a food additive for its taste and scent. In particular, CZE expressed a selective Ela IA, showing an IC 50 value of 104.9 ± 2.1 mg/mL, providing evidence in favour of the use of this herbal product also for skin care.
Concerning Tyr IA, the IC 50 values calculated for the nine active extracts ranged from 19.3 ± 1.2 to 164.3 ± 25.5 lg/mL (Figure 1(B)), and the highest IA was shown by CSI, GGL, RNI, RRO, VVI.
Flowers and bracts of TPL are widely used in southern Europe folk medicine 16 , our results highlighted its strong Tyr IA, indicating a new potential use of this renowned herbal product as cosmetic ingredient. According to Chen et al. 17 , RRO acetone extract exerts Tyr IA with an IC 50 of 181.8 ± 11.0 mg/mL, resulting remarkably less active then the hydroalcoholic extract tested in this study (IC 50 of 19.26 ± 1.16 mg/mL). This difference may be due both to the different assay conditions, particularly incubation time and types of substrate, and to the diverse extraction method employed, since solvent properties strongly affect compounds extraction and consequently the inhibitory effect of a plant extract 18,19 . The same reasons could justify also the significant difference in Tyr IA found for CSI extract by Chen et al. 17 (IC 50 ¼ 232.5 ± 3.3 lg/mL) and the one obtained in this study (IC 50 ¼ 66.04 ± 1.75 lg/mL).
Among the active principles contained in RRO tyrosol and salidroside are known to posses Tyr IA activity 20 . Regarding GGL, it is traditionally and commercially used for skin whitening formulations. The pyranoisoflavan glabridin showed promising anti-tyrosinase activity on melanoma cells and anti-melanogenesis activity on B16 murine melanoma cells. Additionally, reduced pigmentation and inflammation induced by UVB on guinea-pig skins at 0.5% w/v concentration 21 . Licuraside, isoliquiritin and licochalcone A also showed competitive inhibition on monophenolase activity of mushroom tyrosinase 22 , conversely, glabrene and isoliquiritigenin can inhibit both the reactions catalysed by tyrosinase 23 . C. sinensis is widely used in skin care preparations for its peculiar catechines having significant antioxidant, anti-inflammatory and UV-protection activities. Moreover, tea polyphenols, i.e. (-)-epicatechin 3-O-gallate, (-)-gallocatechin 3-O-gallate, and (-)-epigallocatechin 3-O-gallate (EGCG) are known to possess tyrosinase inhibitory potential 24 .
Due to its popularity as cosmetic ingredient, CSI can be considered a further positive control in this screening. On this basis COF and RRO resulted very promising Ela inhibitors, being significantly stronger than CSI. While in the case of TI, GGL, RRO, VVI, and RNI showed an activity comparable to CSI.
Considering the antioxidant properties of polyphenols and flavonoids and their reported activity against several enzymes 25,26 , the total content of these classes of metabolites was evaluated in all the selected samples (Table 1). Pearson correlation test was performed to correlate the percentage of IA (showed by extracts at 50 mg/mL) to the phenolic and flavonoids content respectively.
A moderate positive correlation was found between the IA and the total phenolic content, the highest with TI (r ¼ 0.4965449 and p values ¼ 6.442e-07). A similar correlation trend was found in our previous work 11 , supporting the importance of phenolics in this biological activity. Conversely, for total flavonoid content no correlation was observed with both inhibitory activities. However, the eleven most active extracts resulted interestingly enriched in flavonoids (ranging from 1.47 to 56.47 mg RE/g of extract) and phenolics (ranging from 37.43 to 123.56 mg GAE/g of extract), which indicate also a potential activity as antioxidant agents.

Conclusions
Eleven extracts out of ninety resulted promisingly active against enzymes of cosmetic interest, showing IC 50 values comparable or even lower than positive controls (PMSF and kojic acid). In particular, four out of them inhibited both enzymes, five tyrosinase only and two acted prominently only against elastase. For C. officinarum (aerial parts), C. zeylanicum (barks), R.nigrum (leaves), T. platyphyllos (flowers and bracts) and V. vinifera (leaves) the activity against one or both enzymes was reported for the first time, providing a new perspective for the use of these plants. Among them, C. officinarum (aerial parts) resulted one of the most potent EI, while R. nigum (leaves) and V. vinifera (leaves) showed the highest inhibitory activity against tyrosinase. The plants active against both enzymes (C. sinensis, G. biloba, R. rosea, V. vinifera) are potentially useful to develop cosmetics endowed with both skin-whitening and anti-wrinkles effect. The five plants active against tyrosinase only (G. glabra, R. nigrum, R. officinalis, S. officinalis, T. platyphyllos) are suitable for skin whitening agents, and the two active only against elastase (C. officinarum and C. zeylanicum) are interesting for selective anti-wrinkles cosmetics. Moreover, these plants resulted also enriched in polyphenols and flavonoids, conferring them additional antioxidant properties relevant for cosmetic ingredients and the total phenolic content showed a linear correlation with the enzymatic inhibitory activities.
Further biological and phytochemical studies are ongoing on the selected plants in order to identify the metabolites responsible for the observed biological activities.