陳建煙,李永裕,吳少華
福建農(nóng)林大學(xué)園藝學(xué)院,福建農(nóng)林大學(xué)園藝植物天然產(chǎn)物研究所,福州350002
植物精油是一類植物次生代謝物質(zhì),分子量較小,可隨水蒸氣蒸出,并具有一定揮發(fā)性的油狀液體的總稱,是植物特有芳香物質(zhì)的提取物。植物精油來源豐富,可來源于植物的花、葉、根、樹皮、果實(shí)、種子等植物的各個(gè)部位。植物精油具有極高的應(yīng)用價(jià)值,被廣泛應(yīng)用于醫(yī)療保健、食品工業(yè)、生態(tài)旅游、果蔬保鮮、害蟲防治和日化產(chǎn)品等領(lǐng)域[1,2]。
植物精油具有抗菌消炎、抗癌抑瘤、抗氧化、延緩衰老、抗病毒、防治心血管疾病等多種生物活性,國內(nèi)外對其生物活性的研究日趨活躍。國外對植物精油生物活性的研究起步早,已取得一定成果,并逐步深入到作用機(jī)理的細(xì)胞、分子水平等研究層面。國內(nèi)植物精油研究主要集中在提取技術(shù)、化學(xué)成分分析等方面,較少對其生物活性進(jìn)行研究,植物精油生物活性作用機(jī)理方面的研究更為薄弱。近些年,國內(nèi)對植物精油生物活性的研究雖取得一定成果,但這些成果多集中于抗菌和抗氧化這兩方面上,因此在生物活性方面還具有非常廣闊的研究空間。闡述植物精油生物活性作用機(jī)理,一方面可用于指導(dǎo)國內(nèi)植物精油生物活性研究,另一方面也可為植物精油的藥物開發(fā)利用提供理論基礎(chǔ)。
植物精油的化學(xué)成分極其復(fù)雜,且不同植物或不同部位的精油成分也有差異。植物精油的化學(xué)成分一般分為萜類化合物、芳香族化合物、脂肪族化合物和含氮含硫化合物四種。其中,萜類化合物是精油中最為常見、含量最多的成分,又可以分為單萜、倍半萜和雙萜等;植物精油中常見的萜類化合物有茴香醇(C8H1202)、薄荷酮(C10H180)、金合歡烯(C15H24)、羅勒烯(C10H16)、廣藿香酮(C12H16O4)、月桂烯(C10H16)等,這些化合物常見于桉樹、薄荷、梔子花、廣藿香、月桂等植物精油中。芳香族化合物是植物精油的第二大類化合物,主要是一些酚類、醛類、酮類、酯類、萜源衍生物及苯丙環(huán)類衍生物;植物精油中常見的芳香族化合物有百里香酚(C10H14O)、丁香酚(C10H12O2)、檸檬醛(C10H16O)、肉桂醛(C9H8O)、乙酸肉桂酯(C11H12O2)等,這些化合物在百里香、丁香、肉桂等植物精油中常見。脂肪族化合物在植物精油中含量較小,常見有香茅精油中的異戊醛(C5H10O)、纈草精油中的異戊酸(C5HC10O2)、桂花精油中的乙酸乙酯(C4H8O2)等。含氮含硫化合物在辛香料植物中較多,如大蒜精油中的大蒜素(C6H10S3)、洋蔥精油中的三硫化合物等。
植物精油的幾大類成分具有特殊的生理作用。植物精油中的多種萜類化合物具有抗菌消炎、抗癌抑瘤、抗氧化等作用,其中單萜和倍半萜化合物往往有很強(qiáng)的抗菌作用[3]。百里香醌(thymoquinone)具有很強(qiáng)的抗癌和抗氧化作用;p-百里香素(p-cymene)、β-蒎烯(β-pinene)、長葉烯(longifolene)等都具有一定的抗菌消炎、抗癌抑瘤的作用;而香芹酚(carvacrol)除抗癌效果突出外,還具有一定的鎮(zhèn)痛作用[4,5]。植物精油中的芳香族化合物也具有抗菌、抗病毒、抑制腫瘤生長、鎮(zhèn)痛麻醉等作用,如反式-肉桂醛(trans-cinnamaldehyde)具有很強(qiáng)的抗菌、抗病毒能力,而丁香酚(eugenol)除具有抗菌作用外,還具有抗氧化、抑制腫瘤、麻醉等作用[6-8]。脂肪族化合物在植物精油中含量少,往往不是其主要活性作用成分;其具有特殊香氣,可能對精油整體香氣有特殊的貢獻(xiàn)。含氮含硫化合物較多存在于辛香料植物中,辛香料精油所具有的特殊生理作用可能與含氮含硫化合物密切相關(guān)。
2.1 植物精油抗菌作用機(jī)理
許多植物精油對多數(shù)動植物源病菌有抑制或滅菌作用,對許多革蘭氏陰性和陽性菌均有抑制作用[9]。植物精油對多種人類疾病的病原菌也有抑制作用[10-13],有些精油的抗菌成分還能抑制傷口感染,加速傷口愈合[14]。植物精油的成分和化學(xué)結(jié)構(gòu)[15],及外界條件如低溫低氧低pH等都會影響其抗菌效果[16]。某些精油或成分及精油與抗生素之間的協(xié)同作用或疊加作用也能增強(qiáng)抗菌效果,如香芹酚與其前體p-百里香素、肉桂醛和丁香酚之間、蒿屬植物Artemisia iwayomogi精油與苯唑西林之間都存在協(xié)同作用或疊加作用[16,17]。此外,不同提取方式也會影響精油抗菌效果,如超臨界-CO2萃取法比水蒸氣蒸餾法提取的薰衣草精油抗菌效果更強(qiáng)[18]。
植物精油對病菌的影響方式主要有下列幾種。一是改變病菌細(xì)胞形態(tài)結(jié)構(gòu),如細(xì)胞膜、細(xì)胞壁、細(xì)胞質(zhì)和細(xì)胞器等的結(jié)構(gòu),造成細(xì)胞不可逆的損傷[13,19-22],最終導(dǎo)致病菌細(xì)胞瓦解而死亡。二是改變菌絲體形態(tài)結(jié)構(gòu)誘發(fā)菌絲體溶解而導(dǎo)致病菌的死亡[20-22]。三是降低或抑制分生孢子的產(chǎn)生和萌發(fā),或破壞其受精作用[13,20,22],降低或阻斷病菌后代繼續(xù)為害的可能。丁香、百里香、甜橙外果皮、山香(Hyptis suaveolens L.)、牛至、紫蘇、天竺葵、薰衣草、孔雀草、迷迭香、月桂、小茴香、紫玉蘭等植物精油,具有改變相應(yīng)病菌細(xì)胞或菌絲體的形態(tài)結(jié)構(gòu)或作用于分生孢子,因此都具有一定的抗菌效果[10,11,13,19-24]。植物精油中的芳樟醇(linalool)、反式-肉桂醛、檸檬醛(citral)、香芹酚、百里香酚(thymol)、γ-松油烯(γ-terpinene)、p-百里香素、α-蒎烯(α-pinene)、乙酸龍腦酯(bornyl acetate)、樟腦(camphor)、1,8-桉油素(1,8-cineole)、β-蒎烯、丁香酚、側(cè)柏酮(thujone)、香葉醇(geraniol)、紫蘇醛(perillaldehyde)等成分具有一定的抗菌能力[16],上述多種植物精油具有抗菌作用可能與其富含這些成分中的一種或多種有關(guān)。植物精油及成分引起病菌死亡具體作用途徑有所不同。Zhu等[25]認(rèn)為植物精油抗菌成分是一些天然酚類和萜類物質(zhì),因此植物精油抗菌作用方式與其他酚類物質(zhì)相似;酚類物質(zhì)不僅作用于細(xì)胞壁、細(xì)胞膜從而改變細(xì)胞滲透性并釋放出胞內(nèi)物質(zhì)如核糖、谷氨酸鈉等,而且還能干擾膜的正常生理功能如電子轉(zhuǎn)移、營養(yǎng)吸收、蛋白質(zhì)及核酸合成和酶活性等;植物精油抗菌作用機(jī)理可能是這些作用中的一種或多種。Burt[16]則認(rèn)為許多精油及其成分具有的疏水性,能夠分割病菌細(xì)胞膜和線粒體上的脂類,使細(xì)胞滲透性增強(qiáng),導(dǎo)致細(xì)胞內(nèi)含物外滲,最終導(dǎo)致病菌細(xì)胞死亡。生物膜能顯著提高病菌抵御抗菌藥物的能力,而丁香精油的主成分丁香酚會影響病菌細(xì)胞生物膜的形成,降低病菌的抵御能力;丁香酚還能抑制白色念珠菌絲狀物質(zhì)的生長,影響菌絲體的完整,從而發(fā)揮抗菌作用[11]。研究表明,百里香精油具有與抗生素相似的功能,可導(dǎo)致細(xì)菌外膜蛋白質(zhì)結(jié)構(gòu)發(fā)生顯著改變,這種改變可能會影響細(xì)菌的侵入能力,從而發(fā)揮抗菌作用[23]。Chung等[17]認(rèn)為蒿屬植物Artemisia iwayomogi精油的抗菌機(jī)制可能與DNA裂解有關(guān)。由于植物精油成分眾多,且不同成分抗菌機(jī)理可能不盡相同,因此植物精油抗菌機(jī)制通常不是單一的作用方式,而是多點(diǎn)作用機(jī)制[16,24]。目前植物精油抗菌作用機(jī)理研究主要集中在對病菌細(xì)胞超微結(jié)構(gòu)、菌絲體形態(tài)結(jié)構(gòu)和孢子等變化的觀察層面,而較少研究分子水平如蛋白質(zhì)、核酸的抗菌機(jī)理,而這些方面應(yīng)是今后植物精油抗菌作用機(jī)理研究的重點(diǎn)。
2.2 植物精油抗癌、抑制腫瘤細(xì)胞生長作用機(jī)理
許多植物精油對多種腫瘤細(xì)胞和癌細(xì)胞有抑制作用,能抗人體子宮內(nèi)膜癌[26]、口鼻咽癌[22,27]、肺癌[28]、結(jié)腸癌[28,29]、胃癌[30]和白血?。?1-34]等癌細(xì)胞和腫瘤細(xì)胞。精油抗癌的有效成分主要有丁香酚、香葉醇、香芹酚、百里香醌、β-蒎烯、長葉烯、龍腦(borneol)等[4,5,7,29,34,35]。某些植物精油中的片段也具有抗癌抑瘤功效;如從地中海柏木精油中分離純化得到的CHCl3片段對人子宮頸癌細(xì)胞系HeLa細(xì)胞具有很強(qiáng)的細(xì)胞毒性[36]。植物精油還能有效地清除一些可能致癌的物質(zhì),如亞硝酸鹽[37],可在一定程度上預(yù)防癌癥。
植物精油抗癌抑瘤作用通常是通過誘導(dǎo)細(xì)胞凋亡而實(shí)現(xiàn)的。檸檬香茅精油能導(dǎo)致白血病細(xì)胞系HL-60細(xì)胞的表面突起喪失、染色體凝聚和線粒體脊消失,并使Sarcoma-180肉瘤的細(xì)胞核凝縮和破碎,這些都是細(xì)胞凋亡的典型現(xiàn)象[33]。植物精油誘導(dǎo)癌細(xì)胞和腫瘤細(xì)胞凋亡具有多條作用途徑。檸檬香茅精油及其倍半萜物質(zhì)iso-臭根醇(iso-intermedeol)誘導(dǎo)HL-60細(xì)胞凋亡一是通過線粒體途徑:最初6h內(nèi)ROS激增和線粒體膜電位變化可能造成DNA碎裂、細(xì)胞凋亡體形成和sub-G0期DNA亞二倍體增加,ROS激增誘導(dǎo)細(xì)胞色素c(cytochrome c)從線粒體內(nèi)膜上脫離并逐步清除ROS,細(xì)胞色素c在細(xì)胞液內(nèi)的增加又活化caspase-9(半胱天冬酶-9),而caspase-8則被激活的上游端點(diǎn)死亡受體TNFR1、DR4(TRAIL-R1)活化,caspase-8和caspase-9的異常表達(dá)繼而活化下游死亡執(zhí)行蛋白 caspase-3,caspase-3能分裂具有修復(fù)受損DNA功能的PARP1 (ADP核糖聚合酶 1,poly(ADP-ribose)polymerase),使受損DNA無法修復(fù),這可能是導(dǎo)致細(xì)胞最終凋亡的重要因素;此外該精油還能使促細(xì)胞凋亡蛋白Bax從細(xì)胞液遷移入線粒體,而抗細(xì)胞凋亡蛋白Bcl-2的含量保持不變,Bax/Bcl-2比例的改變也可能作用于細(xì)胞凋亡;二是NF-κB途徑,該精油及iso-臭根醇導(dǎo)致轉(zhuǎn)錄因子NF-κB與其抑制調(diào)控蛋白絡(luò)合,使細(xì)胞核內(nèi)NF-κB減少,從而影響相關(guān)基因的轉(zhuǎn)錄,進(jìn)而導(dǎo)致細(xì)胞凋亡[31]。Cha等[27]研究表明,蒿屬植物Artemisia iwayomogi精油誘導(dǎo)口腔癌細(xì)胞系KB細(xì)胞凋亡過程中的線粒體、caspase途徑嚴(yán)格受MAPK(促分裂原活化蛋白激酶)途徑調(diào)控; MAPK途徑也參與百里香醌誘導(dǎo)結(jié)腸癌細(xì)胞凋亡過程[29]。這些研究結(jié)果表明植物精油抗癌抑瘤是一個(gè)相當(dāng)復(fù)雜的過程,其作用機(jī)理仍未研究透徹。
同種精油成分誘導(dǎo)不同癌細(xì)胞凋亡的作用途徑不同。Park等[32]指出丁香酚能導(dǎo)致白血病細(xì)胞系RBL-2H3細(xì)胞凋亡,并觀察到腫瘤抑制因子p53通過ser15位置的磷酸化移向線粒體內(nèi),且這種遷移先于細(xì)胞色素c的釋放和線粒體膜電位的降低,磷酸化后的Phospho-ser 15-p53還能與抗細(xì)胞凋亡蛋白Bcl-2、Bcl-XL在線粒體內(nèi)互作;說明丁香酚是通過Phospho-ser 15-p53移向線粒體并引起一系列的連鎖反應(yīng)而導(dǎo)致細(xì)胞凋亡。而Yoo等[34]則認(rèn)為丁香酚抗前髓細(xì)胞性白血病的機(jī)制可能是:丁香酚通過消耗細(xì)胞內(nèi)的巰基引起氧化還原反應(yīng)失衡產(chǎn)生ROS,ROS導(dǎo)致線粒體跨膜電位喪失引起線粒體滲透性改變,從而使細(xì)胞色素c釋放到細(xì)胞液中激活Caspase-9和Caspase-3的級聯(lián)反應(yīng),這兩者被激活后可能誘導(dǎo)Bax遷入線粒體,而線粒體內(nèi)Bcl-2減少;這些物質(zhì)的相應(yīng)變化及互作共同導(dǎo)致癌細(xì)胞的凋亡。
2.3 植物精油抗氧化、延緩衰老作用機(jī)理
許多植物精油具有抗氧化作用,能減少自由基等物質(zhì)對機(jī)體的傷害,有助延緩衰老。植物精油抗氧化作用還可與其他生物活性互作,共同保護(hù)機(jī)體延緩衰老。其清除自由基作用可能會減少細(xì)胞凋亡和DNA損傷,并能抗動脈粥樣硬化[38,39]。川芎植物(Cnidium officinale和Ligusticum chuanxiong)精油能抑制紫外線(UV-B)傷害造成的受損DNA的遷移,并在細(xì)胞凋亡調(diào)節(jié)基因的控制下,減少p21蛋白的表達(dá),增加細(xì)胞周期蛋白D1(cyclin D1)的表達(dá)水平;川芎精油抑制UV-B引起的DNA損傷和細(xì)胞凋亡作用,可能與其強(qiáng)清除自由基能力有關(guān)。植物精油中具有較強(qiáng)抗氧化能力的成分主要有:乙酸肉桂酯(cinnamyl acetate)、肉桂醛(cinnamaldehyde)、香茅醇(citronellol)、香茅醛(citronellal)、百里香酚、香芹酚、β-月桂烯(β-myrcene)和p-百里香素等[40-43]。
植物精油抗氧化作用主要通過以下途徑:一是直接清除DPPH、H2O2、羥基離子、超氧化物、超氧陰離子等一系列自由基[42,44-50]。二是減少脂質(zhì)過氧化,增加還原能力;如抑制脂氧化酶活性、抗亞油酸氧化、螯合金屬離子和還原鐵離子[49,51-53]等。一種植物精油抗氧化作用可以是上述這些途徑中的一種或多種。Pituranthos chloranthus植物精油可阻止DNA質(zhì)粒中H2O2光解產(chǎn)生自由基誘導(dǎo)的破壞[46]。蓍(Achillea millefolium subsp.millefolium Afan)的精油具有很強(qiáng)的清除DPPH能力,并在Fe3+-EDTA -H2O2脫氧核糖體系中表現(xiàn)出清除羥自由基的能力;同時(shí)還能抑制大鼠肝臟勻漿中非酶類的脂質(zhì)過氧化作用[54]。
2.4 植物精油防治心血管疾病作用機(jī)理
某些植物精油能調(diào)節(jié)血壓、血糖、血脂和膽固醇,縮短凝血時(shí)間,抗動脈粥樣硬化[39,55,56],對心血管疾病有一定的防治作用。精神緊張對心血管有害,會造成冠狀動脈循環(huán)減弱;而薰衣草芳香療法,能降低精神緊張,減少血清皮質(zhì)醇,增加冠狀動脈血流儲備[56],從而保護(hù)心血管系統(tǒng)。
植物精油可通過不同途徑降低血壓。薰衣草精油通過嗅覺途徑進(jìn)入中樞神經(jīng)系統(tǒng)而降低血壓[57],但其進(jìn)入中樞神經(jīng)系統(tǒng)后是直接作用于心血管調(diào)節(jié)中樞或是通過間接途徑調(diào)節(jié)血壓仍未清楚。薔薇木屬植物Aniba canelilla莖皮精油降低血壓是通過松弛血管,而非消除交感神經(jīng)緊張[58]。蘿摩科植物Periploca laevigata根皮精油能有效抑制ACE(Angiotensin I-converting enzyme,血管緊縮素Ⅰ轉(zhuǎn)化酶)活性而降低血壓[59],ACE可通過把無活性angiotensin-I(血管緊縮素-Ⅰ)轉(zhuǎn)化成有收縮血管作用的angiotensin-II和降低有舒張血管作用的緩激肽(bradykinin)而導(dǎo)致血壓升高。植物精油抗動脈粥樣硬化作用與其抗氧化性有關(guān)。Chung等[39]研究表明,苦艾精油抗氧化性可通過降低氧化固醇而改善肝臟脂類代謝;苦艾精油和維生素E能共同抗動脈粥樣硬化,其可能作用方式是抑制人低密度脂蛋白(LDL)的氧化并上調(diào)LDL受體。茴香精油及主成分茴香腦(anethole)抗血栓作用可能通過抑制血小板凝聚、防止血塊凝縮和舒張血管而實(shí)現(xiàn)[60]。研究表明,桃金娘精油是通過增強(qiáng)糖酵解、增加糖原生成和減少糖原分解而降低血糖,而與胰島素效應(yīng)無關(guān);桃金娘精油甚至還會導(dǎo)致低血糖癥,這主要是因?yàn)槟c對葡萄糖吸收減少;桃金娘精油可能是一種α-糖苷酶抑制劑,只對由四氧嘧啶誘導(dǎo)的糖尿病有降血糖功效[55]。
2.5 植物精油抗病毒和消炎作用機(jī)理
植物精油對皰疹病毒(HSV)、流感病毒和肝炎病毒有一定抗性[6,61-63]。植物精油的單萜成分具有抗HSV病毒效果[61],藜蒿精油與脂質(zhì)體的混合物可增加抗病毒效果[62]。植物精油抗病毒一般只能作用于吸附前,而對吸附后的病毒無效[64];精油(茴香、姜、百里香、牛膝草、洋甘菊、檀香精油)可能與吸附前的病毒包膜互作而發(fā)揮作用[65]。但反式-肉桂醛能抑制轉(zhuǎn)錄后病毒蛋白的合成而抗病毒[6],說明植物精油抗病毒有多種作用途徑。
許多植物精油被用于治療皮膚炎癥、過敏性胸膜炎、水腫等炎癥[66-68]。植物精油中的反式-桂皮醛、檸檬醛、龍腦、乙酸龍腦酯、丁香酚、E-橙花叔醇(E-nerolidol)、乙酸肉桂酯、芳樟醇、乙酸芳樟醇、α-蛇麻烯和丁香烯(caryophyllene)及其氧化物等都具有較強(qiáng)的消炎消腫功效[37,67,69-71]。
植物精油消炎的機(jī)理主要是通過阻滯NF-κB途經(jīng)順暢而實(shí)現(xiàn)。正常狀態(tài)下NF-κB與IκB結(jié)合成無活性的復(fù)合體存在于細(xì)胞液中,而受激后IκB會被磷酸化和降解,從而釋放NF-κB并移向核內(nèi),NF-κB在核內(nèi)進(jìn)行一系列的活動引發(fā)炎癥;NF-κB既能決定促炎癥因子如腫瘤壞死因子-α(TNF-α)、白介素-1β(IL-1β)、白介素-6(IL-6)的表達(dá),又能決定調(diào)控發(fā)炎進(jìn)程的誘生型一氧化氮合酶(iNOS)和環(huán)加氧酶-2(COX-2)的活性,間接影響這兩種酶所調(diào)控的促炎癥因子一氧化氮(NO)和前列腺素E2(PGE2)[72]。NF-κB的組成物質(zhì)p50和p65亞基移向核內(nèi)需IκB-α的磷酸化和降解,而濱艾(Artemisia fukudo)精油能保護(hù)IκB-α不被磷酸化和降解,一方面直接阻滯NF-κB釋放,另一方面間接導(dǎo)致核內(nèi)因亞基不足而引起NF-κB合成受阻,從而抑制了促炎癥因子的表達(dá)和相關(guān)調(diào)控酶的活性,最終達(dá)到消炎效果;此外,該精油還能抑制MAPK途經(jīng)中ERK、JNK和p38的活性,而MAPK途經(jīng)可能也參與調(diào)控NF-κB活性,說明該精油具有多種抗炎作用途徑[72]。水翁(Cleistocalyx operculatus)精油抗炎作用的發(fā)揮可能是通過抑制TNF-α、IL-1β及其mRNA的表達(dá)和阻滯NF-κB、p65的核遷移[73]。植物精油具有多種消炎途徑,可能是因?yàn)樗煞志哂胁煌南鬃饔猛緩?。檸檬醛可阻止IκB-α降解、抑制iNOS和NO產(chǎn)生、阻滯p50的核遷移[67];α-蛇麻烯和反式-丁香烯都能減少PGE2、iNOS和COX-2的表達(dá),但α-蛇麻烯能同時(shí)抑制TNF-α和IL-1β的產(chǎn)生,而反式-丁香烯只能減少TNF-α的產(chǎn)生[70]。
2.6 植物精油其他生物活性作用機(jī)理
白芷揮發(fā)油具有鎮(zhèn)痛、鎮(zhèn)靜作用[74],調(diào)整體內(nèi)單胺類神經(jīng)遞質(zhì)含量是其鎮(zhèn)痛機(jī)理之一[75]。芳樟醇鎮(zhèn)痛和抗痛覺敏感歸因于膽堿能系統(tǒng)、阿片系統(tǒng)和多巴胺能系統(tǒng)的興奮,局部麻醉和阻斷N-甲基-D-天冬氨酸受體(NMDA)三方面的作用[37]。Peana等[37]研究表明減少NO產(chǎn)生或釋放,是芳樟醇鎮(zhèn)痛分子機(jī)理的主導(dǎo)或部分原因,這一過程可能還有膽堿能系統(tǒng)和谷氨酸能系統(tǒng)的參與。Batista等[76]研究發(fā)現(xiàn),芳樟醇可以通過抑制促炎癥因子TNF-α和IL-1β而減弱炎癥引起的疼痛,此外芳樟醇還可以通過鈍化神經(jīng)系統(tǒng)超敏反應(yīng)而鎮(zhèn)痛。當(dāng)歸精油具有治療痛經(jīng)的作用,能通過降低子宮平滑肌上PGF2α和PGE2的含量及升高體內(nèi)孕激素水平等途徑緩解痛經(jīng)癥狀[77]。過江藤屬植物L(fēng)ippia dulcis Trev.精油具有解痙作用,這可能與其具有抗組胺能活性和抗膽堿能活性相關(guān)[78]。
近些年來,植物精油生物活性作用機(jī)理研究已取得一定成果,但仍存在一些問題有待突破。一、植物精油是一類天然混合物,其成分眾多、結(jié)構(gòu)復(fù)雜,因此常將植物精油作為整體進(jìn)行研究,而較少進(jìn)行有效成分的篩選及成分間相互作用的研究,因此較不清楚具體是哪種或哪幾種成分發(fā)揮作用,影響了其作用機(jī)理的進(jìn)一步闡明。二、植物精油作用機(jī)理探索較多停留在藥效及細(xì)胞水平的觀察層面,而對其分子機(jī)理的探究較少,限制其機(jī)理的透徹闡明。三、目前植物精油生物活性研究多集中在單一精油上,而對多種精油混合或精油間協(xié)同作用研究甚少,可作為今后研究突破之一。四、植物精油作用機(jī)理研究多建立在體外或動物體試驗(yàn)基礎(chǔ)上,臨床及生產(chǎn)應(yīng)用安全性方面的研究薄弱。五、除上述幾種生物活性外,植物精油還具有發(fā)汗解熱、安神鎮(zhèn)靜、祛痰止咳、平喘、祛風(fēng)健胃和抗抑郁[79]等作用,值得進(jìn)一步研究探索。
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