Chemical Constituents and Health Benefits of Four Chinese Plum Species

Key Lab of Natural Product Chemistry and Application at Universities of Education Department of Xinjiang Uygur Autonomous Region, Yili Normal University, Yining 835000, China Key Laboratory of Crop Physiology, Ecology and Genetic Breeding, Ministry of Education, Jiangxi Agricultural University, Nanchang 330045, Jiangxi, China College of Agronomy, Jiangxi Agricultural University, Jiangxi Key Laboratory for Post-harvest Technology and Nondestructive Testing of Fruits & Vegetables, Collaborative Innovation Center of Post-harvest Key Technology and Quality Safety of Fruits and Vegetables in Jiangxi Province, Nanchang 330045, China Department of Physiology and Biochemistry, Cholistan University of Veterinary and Animal Sciences (CUVAS), Bahawalpur 63100, Pakistan


Introduction
Prunus is a large genus (340 species) comprising variable habit of plants belonging to family Rosaceae of flowering plants.
e major representatives of the genus Prunus include plums, cherries, peaches, apricots, and almonds [1]. Over 500 listed phytochemicals have been isolated from this single genus so far from this genus. In the past few decades, plums have been explained as health-promoting foods due to their anti-inflammatory actions, improving neurological disorders and strong antioxidant nature mainly because of phenolic compounds and anthocyanins [2].
Prunus species are rich in biologically active ingredients, e.g., apigenin bears strong anticancer properties as a study by Jiang and colleagues showed that it inhibits hypoxia-inducible factor-1 (HIF-1) and vascular endothelial growth factor (VEGF) expression in human ovarian cancer cells [3]. Moreover, Apigenin inhibited tumorigenesis as examined by Matrigel plug assay and chorioallantoic membrane assay (CAM assay) [3]. Similarly, another active ingredient luteolin in Prunus species, which is also reported to show strong anticancer activity by inhibiting the growth and invasion of prostate cancer (PC3) cells, is used to check lung metastasis in an in vivo study, and hence, it is highlighted that luteolin targets E-cadherin and may be a useful way to cure invasive prostrate cancer [4]. Chrysin shows anticancer effects by potentially inhibiting HIF-1α [5], while kaempferol, quercetin, and multiforin extracted from Prunus species showed strong antioxidant potential in the DPPH radical scavenging assay [6][7][8]. Traditionally, plums are dried and processed for a prolonged shelf life of plums. Considering the dried fruits in terms of the phytochemical constituents, the presence of antioxidants has been reported to be the maximum compared with other dried fruits or vegetables [9].
Reactive oxygen/nitrogen species (ROS/RNS) are important physiological molecules involved, among others, in cell signaling and host defense [10]. However, an uncontrolled and excessive ROS/RNS production may overwhelm the antioxidant capacity of the cells and lead to deleterious oxidative stress, which is implicated in the pathogenesis of numerous chronic diseases [11]. Various polyphenolic compounds or plant extracts, as natural antioxidants, are regarded as a potential treatment option in prophylaxis and adjunctive therapy of CVD, and their mechanisms of action, including possible interactions with ROS/RNS, are closely looked into the restriction of onset of oxidative stress-mediated disease [12].

Aims and Objectives.
As the genus Prunus is one of the largest ones, there is no authentic and detailed study describing the biological activities of its constituents in four Chinese Prunus species, viz., P. cerasifera, P. domestica, P. salicina, and P. spinosa. Hence, the current review article will highlight the details about the active phytochemicals and various pharmacological activities already reported.

Chemical Constituents of Four Plum Species
Almost 212 compounds, the majority of which were flavonoids and phenolic acids, isolated from different Prunus species have been classified under different categories and summarized in Tables 1-3 Figure 3).

Biological Activities of Four Plum Species
Various compounds have been isolated from different Prunus species and classified under different categories, viz., polyphenols, flavonoids, anthocyanins, alkaloids, and terpenes. e pharmacological properties of all four Chinese plum species have been discussed in the next sections.
[37]      Journal of Food Quality all of which show high antioxidant activity [50,75]. Excessive presence of various natural and edible pigments in leaves and branches of P. cerasifera make it more demanding in the world market, especially the anthocyanins rich in red edible pigments for beverage industry [29,[76][77][78].

Prunus domestica.
e Cherry plum (Prunus domestica L., Rosaceae) is famous as "Mirabelle plum" or "Myrobalan," and its fruits display a huge variation in size, shape, taste, and appearance [24]. Fresh fruits of P. domestica are processed into dried functional food so that they keep its potential health effects shown by the presence of plenty of phenolics and antioxidants [54]. Prunus kernels which actually are the dried form of the fruits are achieved at 85-90°C for 18 hours [79] and have been utilized in medicine for centuries [63]. In different parts of the world, especially in Southeast Asia, prunes are being administered alone or in combination with other medicines to cure menstrual disorders, leucorrhoea, and debility after the miscarriage [80]. Moreover, plums have a laxative effect due to high fibre contents and higher phenolic contents [81,82]. Certain studies reported that high phenolics may pose positive health effects on the development and strengthening of bones and memory-related issues, reduce inflammations, release constipation, and scavenge ROS [83][84][85][86][87]. e higher antioxidant activity of extracts of P. domestica is mainly because of the presence of phenolic compounds, especially the isomers of caffeoylquinic acid [6,63]. e oral administration of the Prunus fruit extracts (75, 100, 150 mg/kg) to male mice gave much higher learning and memory enhancement [88]. e chlorogenic acid isolated from P. domestica reduces the anxiolytic-like effect in vitro which is linked with anxiety behaviour and provides protection to granulocytes by avoiding ROS efflux [64]. e improvement of bone structure and its biomechanical properties is linked with the usage of higher doses of plums that downregulates the expression of TNF-α in lymphocytes [89], as well as retention of bone calcium ions [86]. e fruit flesh extract of P. domestica inhibited the cell proliferation of breast cancer cells (MCF-7, MDA-MB-453, and MCF-10A cell lines), as well as reduces the toxicity levels in the normal cells [84]. Furthermore, prunes are also found to reduce various other cancers such as colon cancer by inducing apoptosis without any harm to the normal neighbouring cells [90][91][92]. For human liver cancer cells  (HepG2), prunes aid in atntiproliferation activity [90]. Moreover, the polyphenolics in prunes help in reduction in inflammatory markers such as Cyclooxygenase 2 (COX-2) [92]. e fruit extract of dried plums have been reported to be rich in polyphenolic compounds and showed a huge reduction in the inflammatory markers such as nitric oxide and melondialdehyde production in a dose-dependent manner [93]. On the other hand, another class of phytocompounds reported in P. domestica is polysaccharides, which reduces and completely inhibited gastric lesions in a rat model [94]. e fruit extracts of P. domestica showed positive results against peptic ulcer in Wistar albino rats, and it was said that due to the presence of higher amounts of polyphenolics in the fruit juices of plumes, it showed strong antioxidant potential which reduces the oxidative stress and engages various acids to neutralize the corrosive effects of various acids and, hence, appeared as antiulcerogenic [95]  compounds reported in P. domestica minimized the H 2 O 2induced oxidative stress through reducing the intracellular ROS accumulation in granulocytes [96]. Prunes are stated as highly useful to protect against cardiovascular disorders to regulate or modulate blood pressure, prevent atherosclerosis, and boost high-density lipoproteins (HDL) [97]. P. domestica fruit juice (juice concentrate + prune puree + water + 7% fructose) has laxative effects on bowel functions in individuals suffering gastrointestinal issues and increases flatulence [98]. In another study, P. domestica fruit extracts were studied for their antiallergic responses, and it was reported that it reduces the type-I allergic symptoms in mice by adjusting type-1 helper T-cell/type-2 helper T-cell balance and suppression of mast cell degranulation [99].

Prunus salicina Lindl.
e oriental plum (Prunus salicina Lindley, Rosaceae), locally called as Chinese plum, is reported to be a rich source of various pigments such as anthocyanins and polyphenolic compounds. Foods enriched with polyphenolic compounds using oriental plum can improve the symptoms of neurodegenerative diseases by reducing the brain cholesterol levels and upregulation of neurodegenerative-related proteins [100]. e crude extracts of the peels and flesh of P. salicina fruit prepared in acetone have strong anticancer properties as reported for inducing apoptosis in MDA-MB-231 cells [53,101]. Moreover, P. salicina fruits are rich in phenolic antioxidants (82%), of which a nitrogenous compound amygdalin is in higher quantity, which is banned by the FDA as a cancer chemotherapeutic agent [53]. e P. salicina fruits extracts were prepared at various levels of fruit maturity, i.e., immature, partial-mature, and fully mature fruits, and these extracts showed strong anticancer potential against various cancer cell lines, viz. HepG2, Kato11, Hela, U937 lukaemia cells, MCF-7 cells, and MDA-MB-231 human breast cancer cell lines [101]. e mechanism for this anticancer activity was due to the cytotoxic effects of the P. salicina fruit extracts containing polyphenolics that activate apoptotic pathway leading to the programmed cell death [101]. In another study, P. salicina fruit juices showed antiadipogenic effects and reduce inflammations, blood glucose levels, triglycerides, and high-density lipoproteins (HDL) cholesterol levels in obese rats [102]. e   Journal of Food Quality 13 fruit extracts of P. salicina are rich bioactive compounds when mixed with the food supplements which help reduce mite allergic responses [99].

Prunus spinosa L.
Blackthorn or sloe (Prunus spinosa L., Rosaceae) is widely cultivated throughout the world [103]. P. spinosa is resistant to cold, drought, and calcareous soils and represents one of the ancestors of P. domestica [104]. Like all other Chinese prunes, P. spinosa also bears strong biological constituents. Active components of the plant are believed to be polyphenols, including flavonoids and A-type proanthocyanidins, anthocyanins, coumarins, and phenolic acids, forming unique and diversified profiles in particular organs, among which the flowers are the least characterized [30,33,43,47,[105][106][107][108]. e unique composition of phytochemicals in the P. spinosa plant may correspond to the distinctive activity profile reported by traditional Chinese medication systems. Other than traditional Chinese medicine, huge literature reports ethnomedicinal and ethnopharmacological uses of P. spinosa, which showed its potential benefits to cure various diseases [1,32]. P. spinosa fruits and its juice could be considered as a valuable source of antioxidant compounds for nutritional supplementation, as well as of herbal medicine [18]. P. spinosa L. flowers are a traditional herbal medicine recommended for the adjunctive treatment of oxidative stress-related diseases [65].

Conclusions and Further Prospective
e major phytochemicals have been isolated from four Chinese plum species including polyphenols, flavonoids, and anthocyanins. ere occur huge variations in the total phytochemicals contents in different species, which make these different species to have different biological activities in multiple disease conditions, and even the same variety growing under different edaphic conditions may have different antioxidant capacities. Moreover, 212 known compounds have been reported to be present in these four Chinese plum species, which are helpful to evade chronic oxidative stress-mediated diseases. Moreover, it is suggested to perform some extensive and in-depth studies to find new phytochemicals from these four Chinese plum species which could boost the local industry to fulfill the increasing demands.

Data Availability
No data were used to support this study.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.