Anti-cholinesterase activity remains the primary test used to screen anti-AD drug candidates. Table 1 summarizes the anti-cholinesterase activities reported from Syzygium species. They are often tested in vitro based on Ellman’s method using acetylcholinesterase (AChE) extracted from electric eel and butyrylcholinesterase (BChE) from equine serum. Nonetheless, efforts were made to determine in vitro activity using the parasite Cotylophoron cotylophorum ^[1][34] and the fruit fly Drosophila melanogaster ^[2][35]. Ex vivo AChE activity was also measured, revealing no significant activity from S. cumini ^[3][36]. Apart from that, in vivo AChE and BchE activities were determined in alloxan-induced diabetic rats ^[4][37] and scopolamine-induced memory-impaired rats ^[5][29]. Both studies showed a significant reduction in enzyme activity.

To the ouresearchers' best knowledge, about 10 Syzygium species were investigated for their anti-cholinesterase potential. S. aromaticum (clove) represents the most studied Syzygium species, followed by S. cumini for this activity. Not only alcohol extracts were explored, but essential oils were also examined. Methanol and ethanol were widely used to extract Syzygium species, while leaves were the most studied plant part for this bioactivity. Eugenol compound from clove demonstrated more potent inhibitory activity than its essential oil and extract, indicating this compound might provide major inhibition from this plant ^[6][18]. Darusman et al. reported no significant activity from the leaf extract of S. cumini ^[7][38], but other studies showed a disagreement. A reduction in cholinesterase activity was demonstrated from the essential oil, polyphenol-rich leaf extract as well as the seed extract of S. cumini ^[4][5][8][22,29,37]. The inhibitory activity observed from essential oils of Syzygium coriaceum, S. cumini, S. aromaticum, and S. samarangense proposes their economic importance in therapy and nutrition ^[8][9][10][22,27,39].

The aqueous extract of S. jambos leaves revealed no significant activity ^[11][30]. Similarly, the aqueous extract of clove buds exhibited very weak activity with >250 µg/mL of IC₅₀ ^[12][40], suggesting the unsuitability of water as a solvent for this analysis. Amir Rawa et al. reported the highest number of Syzygium species (Syzygium grande, Syzygium lineatum, S. jambos, and S. polyanthum) exhibiting above 80% cholinesterase inhibition at 200 µg/mL concentration in one experiment ^[13][41]. Among them, S. polyanthum leaf extract showed the lowest IC₅₀ at 8.28 and 6.54 µg/mL against AchE and BchE, respectively. Further fractionation revealed that polar bioactive constituents (tannins and polyphenols) were accountable for the enzyme inhibition. It is noteworthy that the ethanol bud extract from S. aromaticum corrected the AchE rate in cerium chloride (CeCl₃)-induced memory-impaired mice ^[14][42]. This researchtudy revealed a clear association between memory and AchE activity, where improved cholinergic neural transmission alleviated the state of memory in mice. Additional studies are required to confirm the bioactivity from other less known species of Syzygium, but the potency of inhibiting cholinesterase from this genus is well established.

The most common form of diabetes, type 2 diabetes, is generally associated with hyperinsulinemia and insulin resistance. Insulin resistance causes neurodegeneration and impairment in the brain glucose metabolism and cognition, which are also observed in AD patients ^[23][4]. Hyperinsulinemia increases tau phosphorylation and Aß accumulation. Furthermore, neuroinflammation, oxidative stress, mitochondrial dysfunction, and advanced glycation products are evident in diabetic and AD patients ^[23][4]. Both disorders share similar features; medicinal plants that can stimulate insulin secretion would benefit diabetic as well as AD patients.

Zulcafli et al. extensively reviewed the anti-diabetic potential of eight Syzygium species ^[24][50]. The It'sreview reported that the inhibition of enzymes involving carbohydrate metabolisms such as α-glucosidase, maltase, and α-amylase is the most studied mechanism of action in the anti-diabetic potential of Syzygium ^[24][50]. Pertaining to type 2 diabetes, the ethanolic seed extract of S. cumini was shown to stimulate insulin secretion produced by pancreatic-ß cells in alloxan-induced mild and severely diabetic rabbits ^[25][51]. The hydroethanolic extract from S. cumini leaves also improved hyperinsulinemia and insulin resistance by modulating ß-cell insulin release in monosodium L-glutamate (MSG)-induced obese rats ^[26][52]. Additionally, Sahana et al. demonstrated that a reduction in insulin resistance was evident in 30 newly diagnosed type 2 diabetic patients when S. cumini seed powder was administered ^[27][53]. The treatment of high-fat diet/streptozotocin (HFD/STZ)-induced diabetic rats with the aqueous seed extract of S. cumini at 400 mg/kg decreased the levels of serum glucose, insulin, and other diabetic markers ^[28][54].

A study by Shen et al. demonstrated a clear connection between insulin resistance and inflammation in TNF-α-treated FL83B cells ^[29][55]. The suppression of c-Jun N-terminal kinase (JNK) inhibited an inflammatory response as the cells were treated with the fruit extract of S. samarangense. As a result, insulin resistance induced by TNF-α was alleviated via the activation of phosphatidylinositol-3 kinase–protein kinase B (PI3K–Akt/PKB) signaling ^[29][55]. S. aqueum leaf extract, on the other hand, reduced glucose levels, increased insulin secretion, and decreased the collagen deposition associated with its anti-inflammatory and antioxidant responses in STZ-induced diabetic rats ^[30][56]. It decreased the levels of toll-like receptor 4 (TLR-4), myeloid differentiation primary response 88 (MYD88), TNF receptor-associated factor 6 (TRAF-6), and TNF-α correlated to pancreatic inflammatory cell infiltration. Malondialdehyde, a sensitive biomarker of ROS-induced lipid peroxidation, was also reduced ^[30][56].

In another study, vescalagin isolated from S. samarangense ameliorated insulin resistance in high-fructose diet-induced hyperglycemic rats ^[31][57]. Myricitrin isolated from the S. malaccense leaf extract exhibited insulin-like effects by enhancing lipid accumulation, glucose uptake, and adiponectin secretion via the activation of the insulin signaling pathway ^[32][58]. The aqueous extract of Syzygium paniculatum fruits alleviated hepatic insulin resistance at a 100 mg/kg dose by reducing the blockage of the insulin signaling pathway via the improvement of insulin receptor (IR and IRS-1) function in HFD-induced diabetic rats ^[33][59]. In addition, the IR mRNA levels were restored to the control level in type-2 diabetic rats treated with Syzygium jambolanum homeopathic remedies, suggesting improvement in insulin secretion ^[34][60].

Nearly all pathological events, including endoplasmic reticulum stress and autophagy dysfunction, can trigger inflammatory responses in AD ^[23][4]. Moroever, insulin resistance and diabetes have been shown to correlate well with inflammation. For example, the bark extract of S. jambos improved the insulin receptor substrate-2/protein kinase B/glucose transporter-4 (IRS-2/AKT/GLUT4) insulin signaling pathway in the liver while improving glycemic parameters by suppressing inflammation, oxidative stress, and apoptosis in STZ-induced rats ^[35][61]. Inflammation is defined as a physiological defense mechanism by the immune system to combat health hazards, causing pain to occur ^[23][4]. It was demonstrated that microglia accumulate in higher quantities near Aß plaques than in the healthy brain. Amyloid plaques and other factors can activate microglia to initiate neuroinflammation ^[23][4]. Anti-inflammatory drugs enable the central nervous system (CNS) to impede pain signaling in the brain, therefore, reducing inflammation in AD pathogenesis. Recent anti-inflammatory activities reported from Syzygium were summarized in Table 2.

The levels of pro-inflammatory mediators such as IL-6, IL-1β, and TNF-α were generally measured to determine the anti-inflammatory activity (Table 2). The inflammation was induced by a toxic chemical or drug such as alloxan and STZ to stimulate inflammatory diabetes in model rats. LPS- or HFD-induced inflammation in diabetic rats was also conducted to observe the anti-inflammatory potential of Syzygium. So far, one study has demonstrated a close correlation between inflammation and memory loss in AD model rats. The memory-related learning ability of Aβ_1-40-infused AD model rats was improved as pro-inflammatory TNF-α and lipid peroxide (LPO) were suppressed when S. cumini seed extract was administered ^[36][62]. The leaf, fruit, pulp, and seed extracts of S. cumini exerting anti-inflammatory activity suggested that almost all parts are bioactive (Table 2).

S. malaccense leaf extract exerted neuroinflammatory protection against LPS-induced neuroinflammation on murine BV-2 microglial cell lines by reducing nitric oxide production ^[37][63]. Nitric oxide (NO) is one of the pro-inflammatory mediators released by microglia; reducing the NO levels can minimize immune outrage caused by microglia ^[23][4]. Other less studied Syzygium species have also been observed to exert anti-inflammatory activities, including Syzygium caryophyllatum, Syzygium mundagam, Syzygium calophyllifolium, and S. samarangense (Table 2).

S. aromaticum (clove) is indeed the most studied Syzygium species for its antioxidant capacity. Its economically important essential oil source has brought many interests toward its applications. Alfikri et al. reported that clove produced the best essential oil ingredient at the flowering stage and the most efficient source of antioxidants when the trees are young. ^[60][86]. Meanwhile, Teles et al. demonstrated that eugenol (the primary compound of clove oil) exhibited higher antioxidant activity than its essential oil ^[61][87]. Various plant parts of the clove have been examined, especially its bud essential oil (Table 3). As for S. cumini and S. malaccense, their dried peel powders showed higher phenolic compounds, anthocyanin content, and antioxidant activity than their freeze-dried extracts, which can be pharmacologically relevant to their food applications ^[62][88].

Oxidative stress is highly apparent when redox circuitry is disrupted, and macromolecular damage occurs, leading to an imbalance in the pro-oxidant and antioxidant levels ^[57][83]. The overproduction of RIS, such as hydrogen peroxide (H₂O₂), hydroxyl radical (HO), and NO observed in the AD brain, can trigger severe oxidative stress. Various factors can contribute to excessive RIS, including mitochondrial dysfunction, high levels of cytochrome oxidase, and Aß peptide chelation by redox-active metal ions ^[23][57][4,83]. Moreover, the downregulation of the expression and activity of antioxidant enzymes such as dehydrogenase complexes is evident in AD, causing biomolecular damage (lipids, proteins, and DNA) and neuronal death ^[23][4]. Antioxidants help scavenge free radicals and balance the production of RIS to reduce oxidative damage ^[58][84].

Syzygium is undeniably a great source of antioxidant agents due to the presence of polyphenols, tannins, and flavonoids ^[58][84]. The evidence of antioxidant activities from Syzygium species was summed up in Table 3. Compared to other biological activities, the antioxidant potential is the most comprehensively explored in various Syzygium species, including less studied Syzygium cymosum, S. paniculatum, and S. caryophyllatum. They not only scavenge free radicals but also provide protective effects against other toxicities. For example, CeCl₃-induced neurotoxicity in the brains of rats was improved by the antioxidant capacity of S. aromaticum to restore RIS levels, which alleviated the cholinergic neural transmission and the state of memory in mice ^[14][42]. In another study, the ability of antioxidants to maintain genomic stability and slow down aging was demonstrated in the Caenorhabditis elegans nematode model ^[59][85]. The essential oil of S. aromaticum exerted antioxidant potential by inducing the expression of SOD-3 or GST-4 (antioxidant enzymes) and DAF-16/FOXO nuclear translocation from the cytoplasm to promote longevity in C. elegans ^[59][85].