Abbreviations: HUVECs, human umbilical vein endothelial cells; DPPH, 2,2-Diphenyl-1-picrylhydrazy; ABTS, 2,20-Azinobis (3-ethylbenzothiazoline-6-sulphonic acid) diammonium salt; FRAP, ferric reducing antioxidant power; iNOS, inducible nitric oxide synthase; PMA, phorbol myristate acetate; COX-2, cyclooxygenase-2; VCAM-1, vascular cell adhesion moelcule-1; NF-κB, nuclear factor kappa-B; ACE, angiotensin converting enzyme; HEL, human embryo lung; PAS, periodic acid-Schiff; bw, body weight.

4.1. Antimicrobial Activity

Various parts of many species of Parkia have good antimicrobial activities. They are most active against S. aureus and E. coli (Table 3). So far, there is still no clinical study conducted on the plants investigating the activity. Enormous reports have been made on antimicrobial activity of different parts of P. biglobosa such as leaves ^{[23][42][131][186][187][188][189]}[23,65,131,132,133,134,135], stem barks ^{[42][45][79][89][91][186][188][189][190][191]}[31,43,45,65,67,131,134,135,136,137], seeds ^[192][138], roots ^{[44][82][193]}[34,38,139] and pods ^[131][194][133,140]. Furthermore, the stem barks and leaves of P. clappertoniana aqueous and methanol extracts investigated on some Gram-positive and Gram-negative bacteria revealed that both stem barks and leaves were effective in all tested organisms, but methanol extract was more potent ^[49][71]. The ethanol extract of both leaves and barks demonstrated growth inhibitory effects on multi-drug resistant Salmonella and Shigella isolates ^[51][73]. The ethanol extract of P. platycephala seeds tested against six bacteria strains and three yeasts showed no antimicrobial activity ^[195][141]. However, lectin obtained from the seed was reported to significantly enhance antibiotic activity of gentamicin against S. aureus and E. coli multi-resistant strains due to interaction between carbohydrate-binding site of the lectin and the antibiotic ^[196][142]. In P. speciosa, the water suspension of the seeds displays some remarkable inhibitory activity against bacteria isolated from the moribund fishes and shrimps—S. aureus, A. hydrophila, S. agalactiae, S. anginosus and V. parahaemolyticus—but no detectable activity against E. coli, V. alginolyticus, E. tarda, C. freundii, and V. vulnificus ^[132][143]. The methanol, chloroform, and petroleum ether extracts of the seeds demonstrate growth inhibitory effect against H. pylori ^[137][144], while the ethyl acetate extract against E. coli, but no effect on S. typhi, S. sonnei, and S. typhimurium ^[137][144]. The antimicrobial activity of P. speciosa is attributable to the presence of cyclic polysulfides 85 and 92–94 in the seeds [126]. However, possible mechanism of the polysulfides was not elucidated. Both pod extract and its synthesized silver nanoparticles exhibit antibacterial activity, with the latter shows higher activity against P. aeruginosa ^[134][145]. A similar antibacterial activity is also seen with aqueous extract of P. speciosa leaves and its silver nanoparticles against S. aureus, B. subtilis, E. coli, and P. aeruginosa ^[197][146]. Its bark methanol extract inhibits the growth of Gloeophyllum trabeum, but not Pycnoporus sanguineus, which effect is not seen with both sapwood and heartwood of the plant ^[135][147]. Its ethyl acetate extract of the peel also shows four times higher activity against S. aureus and three times higher against E. coli than streptomycin, but n-hexane extract exhibits lower activity ^[133][148]. Various parts of P. timoriana inhibit growth of B. cereus, V. cholerae, E. coli, and S. aureus ^[198][149]. Its leaf extract exhibits significant growth inhibitory effect against E. coli, V. cholerae, S. aureus, and B. cereus ^[199][150], while its gold and silver nanoparticles from dried leaves inhibit S. aureus growth. The activity is believed to be attributable to the accumulation and absorption of the gold and silver nanoparticles into S. aureus cell wall ^[140][151]. The methanol extract and semi-polar fractions (chloroform and ethyl acetate) of the bark demonstrate significant inhibitory effects against Neisseria gonorrhoeae. The chloroform extract shows the best activity ^[76][97]. The aqueous extract of the seed, leaf and skin pod also possess antimicrobial activity ^[141][152]. Acetone, ethanol and aqueous extracts of P. biglandulosa stem bark were among the plant extracts that show the highest antimicrobial activity against bacteria and fungi ^[47][69] as well as plant pathogenic bacteria ^[200][153]. The methanol extract of the leaves also shows remarkable growth inhibition against E. coli, P. aeruginosa, and S. aureus ^[142][154]. Investigation on P. bicolor indicates that ethyl acetate, ethanol, and aqueous extracts of the leaves demonstrate a concentration-dependent growth inhibitory effect against some Gram-positive of bacteria such as E. coli, S. aureus, P. aeruginosa, A. niger, B. cereus, and a fungus, C. utilis [23]. Methanol, ethyl acetate, and water extracts of the root also exhibit different degrees of inhibition against some common human pathogenic bacteria including C. diphtheriae, K. pneumoniae, P. mirabilis, S. typhi, and S. pyogenes [28]. The possible mechanism of the antimicrobial activity of Parkia plants are yet to be determined. However, terpenoids from the plants could induce lipid flippase activity in the bacterial cellular membrane which then enhances membrane damage for a better cell penetration ^[201][155]. Other possible actions could be by damaging bacterial protein, inhibiting DNA gyrase and DNA synthesis, which are yet to be confirmed in further studies. Collectively, it can be concluded that the antimicrobial properties of Parkia plants depend on the species and parts of Parkia as well as solvent (polar and non-polar). Most of the published report are in in vitro evaluations, which do not assure the same outcomes in animal models and clinical setting. In the rise of resistant pathogenic bacteria to antimicrobial therapy, it is an urgent need to develop new antimicrobial agents, and phytoconstituents from plants like Parkia could be good candidates.

4.2. Antidiabetic Activity

P. speciosa is the most studied among other species for antidiabetic activity. Six studies comprising three in vitro and three in vivo studies have demonstrated hypoglycemic activity of the plant, but no clinical study has been conducted. Most of the studies have studied the activity in the seeds and pods ^{[120][144][145][202]}[120,156,157,158]. Pericarps from P. speciosa show significant inhibitory activity (IC₅₀ 0.0581 mg/mL; 89.46%) against α-glucosidase ^[202][156], an enzyme responsible for breaking down starch and polysaccharide into glucose ^[203][159]. The seeds also show inhibitory activity but at lower percentage (45.72%) ^[202][156]. In another study, the ethanol extract of the rind had the highest α-glucosidase inhibitory activity followed by the leaf and seed with IC₅₀ of 4596 ppm, 54,341 ppm, and 67,425 ppm, respectively as compared with acarbose having 162,508 ppm ^[145][158]. An in vivo study conducted on both seeds and pods of P. speciosa in alloxan-induced diabetic rats, indicated that only chloroform extract of both pods and seeds exhibited strong glucose-lowering activity. The hypoglycemic activity of the seeds was higher than that of the pods (57% and 36%, respectively) ^[144][157]. A mixture of 66% β-sitosterol 60 and 34% stigmasterol 61 is believed to be responsible for the hypoglycemic effect of the seeds—demonstrated 83% decrease in blood glucose level (100 mg/kg body weight) compared to glibenclamide (111% at 5 mg/kg bw) [120]. Similarly, stigmast-4-en-3-one 63 was identified as the compound responsible for the 84% reduction in blood glucose level at 100 mg/kg bw of the pod extract of P. speciosa [123]. Both compounds (β-sitosterol and stigmasterol) are believed to reduce blood glucose level by regenerating remnant β-cells and stimulating insulin release ^[146][160] via augmentation of GLUT4 glucose transporter expression ^[147][161]. Stigmasterol is also reported to inhibit the ß-cells apoptosis ^[204][162]. In other Parkia species, methanol crude extracts and fractions of P. timoriana pods showed significant α-glucosidase and α-amylase inhibitory activities in streptozotocin-induced diabetic rats. Ethyl acetate fraction had the highest α-glucosidase inhibitory and moderate α-amylase inhibitory activities, with maximal reduction in blood glucose level back to normal observed on day 14 at the dose of 100 mg/kg body weight [18]. α-Amylase functions to hydrolyze starch into maltose and glucose ^[205][163]. Bioassay-guided chemical investigation of the most active ethyl acetate fraction revealed epigallocatechin gallate 4 and apigenin 14 were responsible for the antidiabetic activity [18]. Oral administration of P. biglobosa methanol and aqueous extracts of fermented seeds exhibited different degrees of hypoglycemic effects on fasting plasma glucose when tested on alloxan-induced diabetic rats after four weeks ^[206][207][164,165]. Oral administration of P. biglobosa seeds methanol extract (1 g/kg body weight) lowered blood glucose level by 44.1% at 8 h as compared with glibenclamide (37.9%) in alloxan-induced diabetic rats. Its chloroform fraction exerted maximum glucose-lowering effect (65.7%), while n-hexane fraction had the lowest (4.7%) ^[86][39]. As previously mentioned, similar underlying mechanism of the hypoglycemic activity of the plant species is suggested which is via an improvement in pancreatic islet functions to release insulin, while abolishing insulin resistance ^[208][166]. For future study directions, investigations on the effects of the plant extracts and pure compounds on insulin release and signaling pathways that might be involved in the glucose-lowering properties could be conducted. The compounds should also be studied clinically.

4.3. Anticancer Activity

Cancer is one of the diseases that cause death of millions worldwide. Dietary intake of raw seeds was also reported to significantly lower the occurrence of esophageal cancer in southern Thailand ^[209][167]. The methanol extract of P. speciosa seeds exhibited a moderate antimutagenic activity in the Ames test ^[210][168], but weak activity in Epstein–Barr virus inhibitory assay ^[211][169]. The methanol extract of the seed coats demonstrated selective cytotoxicity against MCG-7 and T47D (breast cancer), HCT-116 (colon cancer) and HepG2 (hepatocarcinoma) cells, while its ethyl acetate fraction only showed selective cytotoxicity against MCF-7, breast cancer cells ^[152][170]. Substances that enhance mitogenesis of lymphocytes may be useful as antitumor or antiproliferative and immunomodulator agents ^[212][171]. Lectin obtained from the P. speciosa seeds exerted mitogenic activity in both rat thymocytes and human lymphocytes by stimulating the incorporation of thymidine into DNA cell, which activity was comparable to the known T-cell mitogens like pokeweed mitogen, concanavalin A and phytohemagglutinin ^[6][213][6,172]. Lectins isolated from the seeds of P. biglandulosa and P. roxburghii have demonstrated antiproliferative effect on murine macrophage cancer cell lines—P 388DI and J774. The seed extract P. roxburghii also inhibits the proliferation of B-cell hybridoma cell line, HB98 ^[150][173], and HepG2 cells without affecting the normal cells ^[90][44]. The monosaccharide saponins 52–55 isolated from P. bicolor root also exhibit moderate antiproliferative effect IC₅₀ ranging from 48.49 to 81.66 µM [118]. To researchers' knowledge, the anticancer effects of Parkia extracts were only investigated in cell lines—limited to cell growth inhibition—not yet studied in in vivo models. An in vitro study on human cancer cell lines has shown that the methanol extracts of P. biglobosa and P. filicoidea exhibit different degrees of antiproliferative activities on T-549 and BT-20 (prostate cancer), PC-3 (acute T cell leukemia Jurka), and SW-480 (colon cancer) at concentrations of 20 and 200 µg/mL. P. biglobosa also exhibits higher cytotoxic activity against all types of cancer cell lines used compared with P. filicoidea ^[153][174]. The antitumor property could be attributable to the antiangiogenic activity of some species of Parkia such as P. biglandulosa and P. speciosa extracts ^[152][167][170,175]. Angiogenesis or neovascularization is involved in metastasis of solid tumors. Methanol extract of the P. speciosa fresh pods was reported to exhibit antiangiogenic activity by more than 50% inhibition of microvessel outgrowth in rat aortae and human umbilical vein endothelial cells forming capillary-like structures in Matrigel matrix. The effect may be attributable to the ability of the compounds in the extract to form vacuoles in the cells ^[152][170], which is essential in maintaining the viability of the cells, therefore beneficial in the treatment of cancer owing to its capacity to prevent tumor neovascularization ^[214][176]. The plant bioactive compounds could also possibly increase apoptotic signaling pathway by elevating caspase activation as similarly shown by the same compounds in other plant species ^[215][177], as well as a direct inhibition on DNA synthesis, related to the ability to inhibit the expressions of several tumor- and angiogenesis-associated genes. Future studies should explore on the possible mechanism of action that are responsible for the anticancer activity. Additionally, future research on human studies is needed to confirm the outcomes seen in the laboratories.

4.4. Antihypertensive Activity

Antihypertensive activity of P. biglobosa seeds has been demonstrated in both animals and human. Only a clinical study was conducted which observed lower blood pressure, blood glucose and heart rate, high level of magnesium as well as improved lipid profile in patients with hypertension consuming fermented seeds of P. biglobosa in comparison with the non-consumption group ^[159][178]. Administration of 1.9 mg/mL of seed extract of P. biglobosa lowers the arterial blood pressure level in a rat model, possibly due to its ability to slow down the heart rate ^[216][179] and to induce vascular relaxation ^[158][180]. The latter effect is also seen with roasted seeds of the plant ^[158][180]. Other than the seeds, P. biglobosa stem bark aqueous extract also demonstrates good hypotensive effect in adrenaline-induced hypertensive female rabbits, which effect is comparable to antihypertensive drugs, propranolol and nifedipine ^[157][181]. The hypotensive properties of P. biglobosa could be owing to its main phytochemicals−phenolics and flavonoids. Catechin and its derivatives are among the most common compounds detected in the plant. These compounds promote vasorelaxation ^[217][182] by modulating nitric oxide availability ^[218][183] and inhibiting angiotensin-converting enzyme (ACE) ^[219][184], in addition to a reduction in oxidative stress ^[220][185], leading to blood pressure-lowering effects of the plant extract. The fermented seeds also decrease plasma triglyceride and cholesterol levels in Tyloxapol-induced hyperlipidemic rats ^[221][186], and platelet aggregation ^[222][187]. P. speciosa empty pod extract has been reported to prevent the development of hypertension in rats given L-NG-nitroarginine methyl ester (L-NAME), a nitric oxide synthase inhibitor, possibly due to its ability to prevent nitric oxide loss [122], which is dependent on the availability of endothelial nitric oxide synthase ^[223][188], as well as to inhibit ACE and oxidative stress and inflammation [112]. Both oxidative stress and inflammation are known to play important roles in the pathogenesis of hypertension (Siti et al. 2015). Active peptide obtained from hydrolyzed P. speciosa seeds displays ACE-inhibitory effect, ranging from 50.6% to 80.2%, which effect is not observed in the non-hydrolyzed seeds, possibly due its long and bulky structure ^[155][156][189,190]. However, the study of Khalid and Babji (2018) has demonstrated that the aqueous extract of the seeds also possesses ACE inhibitory activity ^[154][191]. These studies suggest that the blood pressure-lowering effect of the P. speciosa is most likely due to its ACE inhibitory property and nitric oxide regulation, attributable to its rich contents of polyphenols and the presence of peptide. Future studies should involve isolation of the active compounds which have a potential to be developed as a specific inhibitor of ACE. Other possible mechanisms—specific receptor antagonism, such as adrenoceptors and calcium channels, or modification of signaling pathways—of blood pressure-lowering effects of the plant extract or compounds should be explored further.

4.5. Antidiarrheal Activity

The antidiarrheal effect of Parkia plants has been investigated using many models such as castor oil- and magnesium-induced diarrhea. The aqueous extract of P. filicoidea stem barks reduces the frequency of stooling in rats with castor oil-induced diarrhea, comparable with loperamide ^[161][192]. The aqueous and ethanol extract of P. biglobosa leaves and stem barks also exhibit similar antidiarrheal activity to loperamide, seen as a reduction in stooling frequency and intestinal volume ^{[35][91][160]}[45,59,193]. These effects could be attributable to its inhibitory capacity on the propulsive movement of gastrointestinal tract smooth muscles ^[91][45]. Medicinal plants are believed to exert antidiarrheal activity by enhancing the opening of intestinal potassium channel and stimulating Na⁺/K⁺-ATPase activity, as well as decreasing intracellular calcium concentration, which then promotes gastrointestinal smooth muscle relaxation, leading to diminished diarrhea ^{[224][225][226]}[194,195,196]. The potential of these plants as agents to reduce diarrhea can be explored further in irritable bowel syndrome or chemotherapy-induced diarrhea. Their effects on intestinal mucosal barrier, tight junction proteins and inflammatory cytokines among others can be examined.

4.6. Antiulcer Activity

The gastroprotective effect of Parkia plants was seen in three species which were P. speciosa, P. platycephala and P. biglandulosa (Table 3). The leaves and seeds of P. speciosa protected against ethanol- and indomethacin-induced gastric ulcer in rats, observed by reductions in the gastric ulcer index and acidity of gastric juice ^[162][163][197,198]. Lesser collagen and fibrotic ulcer were significantly diminished in the extract-treated group ^[163][197]. The ethanol extract of P. platycephala also showed protective effect in gastric mucosal injury models induced by ethanol, ischemia-reperfusion, and ethanol-HCl. However, the extract could not protect against indomethacin-induced gastric lesion ^[164][199]. These plants are rich in flavonoids. The compounds like catechin and quercetin confer antiulcer effects possibly by eradicating the formation of ROS and modulating mucin metabolism in the gastrointestinal tract ^{[227][228][229]}[200,201,202]. Other possible protective mechanisms could be by reducing gastric acid secretion, thereby decreasing gastric acid pH, as seen with cinnamic caffeic, p-coumaric or ferulic acids—the compounds that are present in the plants ^[230][203]. Studies on other possible effects of the extracts or bioactive components such as proton pump inhibition could be of interest. In future, the compounds that are responsible for the protective effects should be identified and the possible protective signaling mechanisms should be elucidated. Moreover, clinical trials can be performed to assess the potential use of Parkia extracts as an antiulcer agent.

4.7. Antianemic Activity

The fermented seeds of P. biglobosa are a rich source of essential minerals such as iron, calcium, thiamine, and phosphorus ^[33][57] which are necessary in forestalling either iron or non-iron deficiency anemia. Therefore, the antianemic capacity of P. biglobosa could be owing to its nutritional composition. The fermented seeds of P. biglobosa in combination with other fermented products were reported to be beneficial in the management of anemia as it increased hemoglobin, red blood cells, white blood cells, and packed cell volume ^[165][204]. The ethanol extract of P. speciosa seeds were also investigated in NaNO₂-induced anemic mice. At doses of 400 and 700 mg/kg, an elevation of hemoglobin levels was noted to 0.92 and 0.82 g/dL, respectively ^[166][205]. The exact mechanism of how P. speciosa acts to decrease anemia is still unclear. It could be due to its rich source of the minerals, particularly the iron ^[212][171]. Another possible mechanism would be stimulation of erythropoiesis process. Both extracts of P. biglobosa P. biglobosa and P. speciosa can be developed as an alternative iron supplement. However, the effectiveness should be evaluated clinically.

4.8. Anti-Inflammatory Activity

Inflammatory reaction is involved in almost all clinical manifestation. Hence, anti-inflammatory activity of certain plant extracts could be of benefit. Anti-inflammatory activity of P. biglobosa stalk ^[172][206], seeds and stem bark [29], P. speciosa pods ^[222][223][187,188] and seeds ^[62][84], as well as P. platycephala seeds ^[175][207] have been reported using various models of inflammation. The protective effects of P. biglobosa is believed via its inhibitions on the lipoxygenase and cyclooxygenase pathways ^[172][206], leading to inhibition of pro-inflammatory cytokine release and stimulation of anti-inflammatory cytokine ^[173][208], as well as increment on membrane stabilization ^[174][209]. While the P. speciosa exerts its anti-inflammatory by downregulating nuclear factor kappa B cell (NF-κB) and p38 mitogen-activated protein kinase (MAPK) pathways ^[222][223][187,188]. It is obvious that the plant bioactive components attenuate inflammation by regulating inflammatory and MAPK signaling pathways, which could lead to reduced formation of inflammatory mediators such as cytokines. To date, no study has identified the anti-inflammatory compounds from Parkia, which warrants further studies on this aspect, either in experimental animals or human studies.

4.9. Antioxidant Activity

Polyphenolic compounds present in plant foods have been reported to be responsible for their antioxidant activity due to their ability to serve as a hydrogen donor and reducing agent (Amorati and Valgimigli 2012). Both fermented and unfermented seeds of P. biglobosa have been reported to contain an appreciable amount of phenolic contents ^[179][184][210,211]. P. timoriana pods are also rich in total phenolic and flavonoid contents ^[178][212]. The antioxidant capacity of the leaves and seeds of P. speciosa has been reported to be relatively lower than that of the empty pods and seed mixture, suggesting that the pods possess higher antioxidant contents than other parts of the plant ^[85][214][37,176]. The difference in geographical location may affect the composition of the antioxidant compounds in plants. It was reported that P. speciosa seeds collected from central Peninsular Malaysia had higher antioxidant capacity than the southern and southwestern regions ^[181][213]. The compounds present in the plants attenuate oxidative stress possibly by activating Nrf2/Keap1 and MAPK signaling pathways, leading to enhanced expressions of Nrf2 and antioxidant enzymes, such as heme oxygenase-1 ^[231][214]. P. speciosa extracts of seed coats and pods could also reduce the risk of hemolysis by inhibiting Heinz body production in the erythrocytes incubated with a hemolytic agent ^[182][215], indicating the ability of the extracts to inhibit oxidative destruction of erythrocyte. The finding suggests a potential of the plant extract to reduce hemolytic jaundice, which warrants further research.

4.10. Other Pharmacological Activities

Other than previously mentioned activities, the P. biglobosa extract has also been demonstrated to have antimalarial effect [11], whereas P. clappertoniana ^[53][75] and P. biglobosa ^[168][216] show nephro- and hepatoprotective effects, respectively (Table 3). P. pendula seeds also enhance wound healing in immunosuppressed mice ^[169][217]. However, extensive studies regarding these effects were not performed. Further studies need to be conducted to explore the possible mechanisms that are involved in the aforementioned beneficial effects.

5. Toxicity

Daily consumption of cooked pods of P. roxburghii does not impose any significant adverse effect ^[232][218]. However, eating raw pods may result in bad breath owing to its rich content in volatile disulfide compounds, which are exhaled in breath and the odor can persist for several hours (Meyer, 1987). Many substances have been identified or isolated from Parkia seed, such as lectins, non-protein amino acids, and alkaloids ^[233][219]. However, no acute mortality and observable behavioral change were recorded at doses up to 2000 mg/kg ethyl acetate fraction of P. roxburghii pod in rats [18]. Investigation on acute and sub-acute toxicity profiles of the aqueous and ethanol extracts of the stem bark of P. biglobosa showed that the oral median lethal dose (LD₅₀) was higher than 5000 mg/kg for both extracts in rats ^[84][36]. However, in another report, LD₅₀ values of the leaves, stems and roots in an acute toxicity study were within the range of 500–5000 mg/kg body weight of fish, suggesting that they are only slightly toxic and, therefore, not potentially dangerous. The adverse effects included respiratory distress and agitated behavior ^[234][220]. Apart from the barks of P. biglobosa, the pods also possess the piscicidal activity that can be used in the management and control of fishponds to eliminate predators ^[234][235][220,221]. Fatty acids and oils identified from the seeds of P. biglobosa and P. bicolor were reported to be non-toxic [22]. The aqueous extract of P. clappertoniana seeds showed no observable maternal and developmental toxicity at 100–500 mg/kg when given orally to Sprague-Dawley rats and mice at different gestational age ([17]. P. platycephala leaves at 1000 mg/kg on the other hand, caused decreases in body mass, food and water consumption in rats. It also shortened the proestrus and prolonged diestrus phases, as well as reduced uterine weight, suggestive of possible alterations on hormonal levels, but no obvious toxicity on other organs ^[98][53]. Oral administration of the leaves of P. speciosa for 14 days showed no significant histopathological toxicity or mortality in rats at up to 5000 mg/kg ^[162][198]. In vitro, the plant pods (100 μg/mL) showed no significant cytotoxic effect on normal cell lines ^[152][170]. Consumption of the seeds up to 30 pieces in a serve does not produce any adverse effects ^[214][176].