Advances in the research of phenolic compounds and their bioactivities in plum fruits

Abstract：Plum is a drupe belonging to the subgenus Prunus (family Rosaceae) , and one of the most important stone fruits in the world. Around the world there are about 30 species of plums, and in China there are 8 species, including Prunus salicina, P. domestica, P. simonii, P. ussuriensis, P. cerasifera, P.americana, P. nigra and P. spinosa. P. salicina plums are mainly for fresh consumption, while P. domestica plums are used for the production of prunes. Each species displays great diversity in shape, size, taste and appearance. Plum fruits are, from a nutritive and dietary point of view, a valuable component of the human diet due to their content in minerals, carotenoids, vitamins, fibre and phenolic compounds.Phenolic compounds are the most important group of secondary metabolites widely distributed in plants, playing important roles in diverse physiological processes such as coloring, flavor formation and stress resistance. In generally, phenolic compounds in plum fruits are determined and characterized by HPLC, using diode array detector (DAD) , photoelectric secondary array (PDA) , ultraviolet detector (UV) , fluorescence detector (FLD) , etc, and then combined with mass spectrometry (HPLC-MS) for primary structure identification. The commonly used is electrosprayionization mass spectrometry (ESI/MS) . The phenolic compounds can be grouped into 4 classes, phenolic acids, flavonols, flavanols and anthocyanins. The phenolic acids detected include neochlorogenic acid, cis-3-caffeoylquinic acid 3-O-pcoumaroylquinic acid, 3-O-p-feruloylquinic acid, cryptochlorogenic acid, 4-O-p-coumaroylquinic acid, n-chlorogenic acid, caffeoyl-glucoses methyl 3-caffeoylquinate, methyl 4-caffeoylquinate, methyl 5-caf-feoylquinate, 5-O-p-coumaroylquinic acid, 5-O-p-feruloylquinic acid, caffeic acid, caffeoylshikimic acid, p-coumaric acid, 3-p-coumaroylquinic acid, cis-3-p-coumaroylquinic acid, 4-p-coumaroylquinic acid, cis-4-p-coumaroylquinic acid, p-coumaroyl-glucoses, methyl 3-p-coumaroylquinate, feruloylquinic acid, feruloyl-glucoses, 3-p-methoxycinnamoylquinic acid, vanillic acid-glucoses 3, 4-dihydroxybenzoyl-glucoses, and esculin. The flavonols are isoquercitrin, rutin, hyperoside, quercetin 3-rhamnoside, quercetin-pentoside, quercetin, quercetin-pentoside-rhamnoside, isorhamnetin-rutinoside, myricetin, and avicularin. The flavanols include catechin, epicatechin, procyanidin B1, procyanidin B2, procyanidin B4, procyanidin B7. The anthocyanins were cyanidin 3-glucoside, cyanidin 3-rutinoside, peonidin 3-glucoside, and peonidin 3-rutinoside. The content of phenolic compounds in plum fruit is usually quantified by mg· kg-1 (fresh weight basis) , and has been shown to vary significantly among different varieties, environmental conditions and test methods. The results of previous studies showed that the content of the same substances varied significantly between different varieties and different periods. For example, a study showed that neochlorogenic acid varied in a range of 0-985.6 mg · kg-1; chorogenic acid 0-57.2 mg · kg-1; coumaroylquinic acid 0-151.6 mg · kg-1; catechin 0-224.7 mg · kg-1; epicatechin 0-97.1 mg · kg-1;rutin 0-119.4 mg · kg-1; procyanidin B1 22.5-328.6 mg · kg-1; procyanidin B2 7.6-101.8 mg · kg-1; cyanidin3-glucoside 0-730 mg · kg-1, and cyanidin 3-rutinoside 0-2 610 mg · kg-1. The content of anthocyanins gradually increases in the peel and flesh as the fruit matures, and procyanidins reach a highest value in both skin and flesh during the stone hardening phase. In addition, phenolics have several pharmacological activities including antioxidant, anticancer, antimutagenic, antidiabetic, anti-inflammatory, and antiHIV activities. The increased interest in plum research has been attributed to its high phenolic content, mostly the anthocyanins, which are known to be natural antioxidants. The effect of immature plum extract on in vitro cancer cells, eg. human hepatocellular carcinoma HepG2 cells, Kato III gastric cancer cells, HeLa human cervical carcinoma cells and U937 leukaemia cells, has been studied and the results show that the extract is effective in inhibiting growth of the cancer cells. Based on these results, the immature plum with its active compound, epicatechin, is expected to be a natural resource for developing novel therapeutic agents for cancer prevention and treatment. After taking the Queen Garnet (a high anthocyanin and high antioxidant plum) plum juice, there was a threefold increase in hippuric acid excretion (a potential biomarker for total polyphenols intake and metabolite) , an increase in urinary antioxidant capacity and a reduction in malondialdehyde, which is a biomarker of oxidative stress. Chlorogenic acid from plum can decrease in anxiety-related behaviors (anxiolytic-like effect) and protect the granulocytes from oxidative stress in mice. Dringking plum juice can also decrease the levels of angiotensin II in plasma and its receptor Agtr1 in heart tissues, suggesting a role of plum polyphenols as peroxisome proliferator-activated receptor-γ agonists, and cardioprotective effects can be achieved by replacing drinks high in sugar content with fruit juice rich in polyphenols in a diet. Meanwhile, polyphenols in dried plums are beneficial to bone health. In this review, we summarize recent research progress on phenolic compounds and their bioactivities in plum fruit in order to provide useful information for the development and utilization of plum fruits and breeding of high-phenolic varieties.