Phenolic Compounds and Biological Activity of Selected Mentha Species

Abstract

Mentha species are widely used as food, medicine, spices, and flavoring agents. Thus, chemical composition is an important parameter for assessing the quality of mints. In general, the contents of menthol, menthone, eucalyptol, and limonene comprise one of the major parameters for assessing the quality of commercially important mints. Building further on the phytochemical characterization of the quality of Mentha species, this work was focused on the composition of phenolic compounds in methanolic extracts. Thirteen Mentha species were grown under the same environmental conditions, and their methanolic extracts were subjected to the LC-MS/MS (liquid chromatography-tandem mass spectrometry) profiling of phenolics and the testing their biological activities, i.e., antioxidant and tyrosinase inhibition activities, which are important features for the cosmetic industry. The total phenolic content (TPC) ranged from 14.81 ± 1.09 mg GAE (gallic acid equivalents)/g for Mentha cervina to 58.93. ± 8.39 mg GAE/g for Mentha suaveolens. The antioxidant activity of examined Mentha related with the content of the phenolic compounds and ranged from 22.79 ± 1.85 to 106.04 ± 3.26 mg TE (Trolox equivalents)/g for M. cervina and Mentha x villosa, respectively. Additionally, Mentha pulegium (123.89 ± 5.64 mg KAE (kojic acid equivalents)/g) and Mentha x piperita (102.82 ± 15.16 mg KAE/g) showed a strong inhibition of the enzyme tyrosinase, which is related to skin hyperpigmentation. The most abundant compound in all samples was rosmarinic acid, ranging from 1363.38 ± 8323 to 2557.08 ± 64.21 μg/g. In general, the levels of phenolic acids in all examined mint extracts did not significantly differ. On the contrary, the levels of flavonoids varied within the species, especially in the case of hesperidin (from 0.73 ± 0.02 to 109. 39 ± 2.01 μg/g), luteolin (from 1.84 ± 0.11 to 31.03 ± 0.16 μg/g), and kaempferol (from 1.30 ± 0.17 to 33.68 ± 0.81 μg/g). Overall results indicated that all examined mints possess significant amounts of phenolic compounds that are responsible for antioxidant activity and, to some extent, for tyrosinase inhibition activity. Phenolics also proved to be adequate compounds, together with terpenoids, for the characterization of Mentha sp. Additionally, citrus-scented Mentha x villosa could be selected as a good candidate for the food and pharmaceutical industry, especially due its chemical composition and easy cultivation, even in winter continental conditions.

Keywords: LC–MS/MS; Mentha sp.; antioxidant activity; phenolic compounds; tyrosinase inhibition activity.

Figure 1

The number of studies on Mentha species in the last twenty years (Scopus, accessed on 28 December 2020).

Figure 2

Heatmap of the levels of phenolic compounds in different Mentha species. LUT: luteolin; KAEM: kaempferol; 3HBA: 3-hydroxybenzoic acid; API: apigenin; VA: vanillic acid; RUT: rutin; 4HBA: 4-hydroxybenzoic acid; NAR: naringenin; SaA: salicylic acid; 23DHBA: 2,3-dihydroxybenzoic acid; SaAG: salicylic acid 2-O-β-glucoside; HESP: hesperidin; pCA: p-coumaric acid; FA: ferulic acid; pMCA: p-methyl coumarate; CA: caffeic acid; RA: rosmarinic acid; ChA: chlorogenic acid; tCA: trans-cinnamic acid; GA: gallic acid; QUE: quercetin.

Figure 3

Volatile profiles of selected Mentha species.

Figure 4

Heat maps of the headspaces (A) and essential oils (B) in thirteen different Mentha species. LIN: linalool; LINAC: linalyl acetate; MYR: myrcene; PIPOX: piperitenone oxide; LIM: limonene; CAR: carvone; APIN: α-pinene; BPIN: β-pinene; MEN: menthone; IMEN: isomenthone; TERP: terpinen-4-ol; ATHU: α-thujene; GTERP: γ-terpinene; PCYM: p-cymene; CIN: 1,8-cineole; THY: thymol; PIP: piperitenone; DHC: dihydrocarvone; TERPAC: α-terpinyl acetate; MEH: menthofuran; MON: menthone; PUL: pulegone.

Figure 5

(A) Correlation matrix between phenolics, total flavonoids (TFC), total phenolics (TPC), and antioxidant (DPPH) and tyrosinase (TIA) activity in Mentha species. (B) Correlation matrix between terpenoids and DPPH and TIA in Mentha species. Red asterisks indicate the significance of the correlation (* p < 0.05; ** p < 0.01; *** p < 0.001). VA: vanillic acid; RUT: rutin; 3HBA: 3-hydroxybenzoic acid; API: apigenin; TIA: tyrosinase inhibition activity; LUT: luteolin; KAEM: kaempferol; ChA: chlorogenic acid; tCA: trans-cinnamic acid; QUE: quercetin; GA: gallic acid; pMCA: p-methyl coumarate; HESP: hesperidin; NAR: naringenin; 23DHBA: 2,3-dihydroxzbenyoic acid; SaA: salicylic acid; 4HBA: 4-hydroxybenzoic acid; SaAG - salicylic acid 2-O-β-D-glucoside; FA: ferulic acid; pCA: p-coumaric acid; CA: caffeic acid; RA: rosmarinic acid; TFC: total flavonoid content; TPC: total phenolic content; DPPH: antioxidant activity; PUL: pulegone; PIP: piperitenone; MON: menthone; MEN: menthol; MEF: menthofuran; CIN: 1,8-cineole; TERP: terpinen-4-ol; THY: thymol; MYR: myrcene; PIPOX: piperitenone oxide; DHC: dihydrocarveol; LIM: limonene; CAR: carvone; TERAC: α-terpinyl acetate; LIN: linalool; LINAC: linalyl acetate.

Figure 6

Mentha sp. used in the study.