| Peppermint(Mentha piperita L.) is a herbaceous perennial belongs to Mentha member of the Labiatae family, its essential oil and menthol, which is the major component in essential oil, are widely used in medicine, cosmetics, daily chemicals and so on. Over the years, the research of peppermint is focused on the extraction process, volatile constituents and activity of essential oil, and there has been few scientific studies on polyphenols and its bioactivity. In the paper, response surface method was adopted to optimize technological parameters of ultrasonic extraction of total phenols and flavonoids. On the other, the paper compared the difference of the content of total phenols and flavonoids in different growth period, and researched the polyphenols composition and its bioactivity. It has important practical significance for further developing and promoting diversified utilization of peppermint. The main results of the study were as follows:1. Ultrasonic extraction process for total phenols and flavonoids was optimized by mono-factor experiment and response surface methodology:extraction solvent was methanol, the ratio of material and liquid was 1:40 (m’. V), ultrasonic power was 200 W, ultrasonic temperature was 60℃, ultrasonic time was 45 min.2. The content of total phenolics and flavonoids in peppermint blade was different in different growing period, and the content of total phenolics and flavonoids in reproductive period was higher than in vegetative period, and the difference was significant.3. There were free, soluble glycoside, and insoluble cell-wall-bound phenolics in vegetative period, and in which, the content of free and insoluble cell-wall-bound phenolics was higher. There was no soluble glycoside phenolics, and the content of free phenolics was higher than the content of insoluble cell-wall-bound phenolics in reproductive stage. The content of the total phenols reduced significantly after the extraction of essential oil. The different drying method had some influence on the content of the total phenols and flavonoids. Air drying and microwave drying treatment had little influence on total phenols, and microwave drying method had little influence on total flavonoids.4. The main phenolic acid in peppermint was identified by HPLC method, four phenolic compounds were identified, which was chlorogenic acid, caffeic acid, p-coumaric acid and rosmarinic acid, it was arranged in the order from large content to small content:rosmarinic acid> p-coumaric acid> chlorogenic acid> caffeic acid.5. Peppermint polyphenols had strong capacity of scavenging DPPH radicals, and the capacity of peppermint polyphenols (lmg/mL) was higher than that of Vc and BHT. On the other, peppermint polyphenols showed strong protective effect on β-carotene attacked by free radicals in β-carotene/linoleic acid system, which was better than Vc, and equival with BHT. Peppermint polyphenols showed some scavenging capacity to hydroxyl free radical, to superoxide anion, to nitrite, and had some reducing power to ferric iron, but the control Vc was more effective.6. Peppermint polyphenols had no inhibitory effect on α-amylase, and had some inhibitory effect on α-glucosidase, but the inhibition effect of it was no better than acarbose, which was widely used in clinics.7. Protective effects on DNA damage mediated by OH and ROO’was observed in peppermint polyphenols, and the effect of protection on DNA damage mediated by ROO·, which showed cleardose connection, was better than that mediated by OH*, which was no obvious concentration-response relationship.8. The high dose group of peppermint polyphenols showed protective action on mice induced by CCl4 in some degree. It had obvious protection on enlargement of mice liver, and had some protective effects on enlargement of mice spleen, which was not remarkably. On the other, the increase of concentration of AKP, ALT and AST in mice serum was restrained in the medium and high dose group of peppermint polyphenols, but the inhibition effect was less than that in the positive control(phenoxide drug); The high dose group of peppermint polyphenols had significant inhibitory effect on elevated TC and TG in the serum of the mice, decreased antioxidant enzyme activity of SOD and CAT in liver, increase of MDA content, and reduction of T-AOC, and which was equivalent with the positive control(phenoxide drug); The mice hepatic cells pathological tissue biopsies showed that mice liver cells in high dose group remained intact with a little spotty necrosis, and had no large necrosis area. It further explained the high dose group of peppermint polyphenols had protective effect on the mice liver damaged by CCl4.
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