Metabolites extracted from microorganisms as potential inhibitors of glycosidases (α-glucosidase and α-amylase): A review

α-Glucosidase and α-amylase are the two main glycosidases that participate in the metabolism of carbohydrates. Inhibitors of these two enzymes are considered an important medical treatment for carbohydrate uptake disorders, such as diabetes and obesity. Microbes are an important source of constituents that have the potential to inhibit glycosidases and can be used as sources of new drugs and dietary supplements. For example, the α-glucosidase inhibitor acarbose, isolated from Actinoplanes sp., has played an important role in adequately controlling type 2 diabetes, but this class of marketed drugs has many drawbacks, such as poor compliance with treatment and expense. This demonstrates the need for new microorganism-derived resources, as well as novel classes of drugs with better compliance, socioeconomic benefits, and safety. This review introduces the literature on microbial sources of α-glucosidase and α-amylase inhibitors, with a focus on endophytes and marine microorganisms, over the most recent 5 years. This paper also reviews the application of glycosidase inhibitors as drugs and dietary supplements. These studies will contribute to the future development of new microorganism-derived glycosidase inhibitors.

Glycosidase inhibitors hitherto have mostly been derived from microbial sources, which can be easily handled on a large scale, as compared to that with plants (Shah et al., 2018). Since the classic α-glucosidase inhibitor nojirimycin, derived from microbial resources, was first reported by Niwa et al. (1970), more and more glycosidase inhibitors have been isolated from microorganisms (Niwa et al., 1970). However, the search for novel microorganisms with new glycosidase inhibitors was previously concentrated on general environment, which has led to microbial resource scarcity (Bisaria et al., 2020). Therefore, the discovery of new inhibitors from uncommon environments, such as the marine ecosystem and endophytic microorganisms, has been the focus in recent years (Davies-Bolorunduro et al., 2021;Karthikeyan et al., 2022).
The marine environment has not been fully explored, and metabolites of marine microbial origins have a more important role in drug discovery (Pathak et al., 2022). These natural products from marine microorganisms have various bioactivities, such as anti-bacterial (Yang et al., 2018), anti-fungal (Sangkanu et al., 2021), anti-viral (Richard et al., 2018), anti-parasitic (Santos et al., 2019), anti-tumor (Yan et al., 2020), and anti-diabetic properties (Elaiyaraja et al., 2018). The survival of bacteria that live in unusual marine ecosystem depends on these active metabolites (Davies-Bolorunduro et al., 2021).
Endophytic microorganisms, which have symbiotic relationships with plants without causing harm, are a prospective source of novel drugs with various bioactivities (Manganyi et al., 2019). Through their interactions with plants, endophytes generate several metabolites with pharmaceutical applications, such as antibacterial , anti-fungal (Deshmukh et al., 2018), anti-viral (Lacerda et al., 2022), anti-cancer (Gangadevi and Muthumary, 2009;Gill and Vasundhara, 2019), anti-oxidant (Toghueo and Boyom, 2019), and anti-diabetic compounds (Agrawal et al., 2022). The utilization of these endophytes for discovering new active metabolites has been increasing, and it ultimately will be of major importance for pharmaceutical, medical, and industrial applications (Lopéz et al., 2019;Qiu et al., 2019).
The aim of the present review was to consolidate the literature on microbial sources of α-glucosidase and α-amylase inhibitors from 2018 to 2022. Peer-reviewed articles were retrieved from searches using the following databases: PubMed, Google Scholar, and Web of Science.

Inhibitors of glycosidases from bacteria
Bacteria are the main traditional sources of glycosidase inhibitors and bacteria with inhibitory activity against glycosidase in recent 5 years are listed in Table 1.

Streptomyces
In the most recent 5 years, most glycosidase inhibitors were still produced by Streptomyces species, and these are very potent. α-Glucosidase inhibitors from Streptomyces costaricanus EBL.HB6 were isolated and purified by Nguyen et al. (2021) in Vietnam. The IC 50 value of purified inhibitors was 9.59 mg/ml. They also optimized cultivation conditions to increase the yield of inhibitors. However, very few Streptomyces strains with glycosidase inhibitory activity have originated from the general environment in recent years, and most of them are derived from the marine ecosystem and endophytic microorganisms.
The deep-ocean interior contains many new Streptomyces strains, which show strong inhibitory activity against glycosidases. Kumar and Rao (2018) confirmed that the extract of the marine organism Streptomyces coelicoflavus SRBVIT13 exhibits remarkable inhibitory activity against α-glucosidase of yeast and mammals in vitro. In vivo, postprandial blood glucose levels decreased after the oral administration of this extract in diabetic rats. The main compound with activity was identified as 2-t-butyl-5-chloromethyl-3-methyl-4-oxoimidazolidine-1-carboxylic acid, t-butyl ester (1). Marine-derived Streptomyces sp.S2A was isolated from the gulf of India by Siddharth and Vittal. The active metabolites were extracted, and the α-glucosidase and α-amylase  (Siddharth and Vittal, 2018). Siddharth and Vittal (2019) further extracted a bioactive fraction from marine Streptomyces sp.SCA29 in India. The fraction was assayed for inhibitory activity against α-glucosidase and α-amylase, and the IC 50 values were 44.26 μg/ml and 53.19 μg/ml, respectively. (3), an acetamide derivative, was also purified based on a bioassay-guided method ( Figure 1). Endophytes from Murraya koenigii comprise a library of novel Streptomyces strains. M. koenigii is a small, deciduous shrub that possesses notable pharmacological effects, such as anti-diabetic, anti-microbial, and anti-diarrheal (Tembhurne and Sakarkar, 2010). Saini confirmed that the endophytic Streptomyces koyangensis strain B025 isolated from M. koenigii has remarkable
Frontiers in Microbiology 04 frontiersin.org inhibitory activity against α-amylase. The active compounds were considered to be phenols (Saini and Gangwar, 2018). For several years, marine endophytic Streptomyces strains isolated from different marine sources comprise one of the most popular research subjects (Chen et al., 2016). The isobutylhexapeptide TXS-2(4) was isolated from the marine Streptomycetes SCSIO 40064 by Chen et al. (2022). They found that this compound significantly inhibited the activity of α-glucosidase, and the IC 50 value was 18.67 ± 1.27 mM. El-Gendy et al. (2022)

demonstrated the effects of a new endophytic
Streptomyces species from the marine Sarcophyton convolutum. The strain displayed α-glucosidase inhibitory activity with an IC 50 value ≥ 84.34 ± 2.25% and α-amylase inhibitory activity ≥ 88.20 ± 1.33%.
Frontiers in Microbiology 05 frontiersin.org Amycolatopsis, Nocardiopsis, Arthrobacter, Saccharomonospora, and other strains in succession. Most of them have come from marine or plant endophytic microorganisms, which are similar to Streptomyces species. An active compound was isolated from Amycolatopsis thermoflava SFMA-103 by Chandrasekhar et al. (2021). The compound was identified as 1-O-methyl chrysophanol (OMC, 5), a class of hydroxyanthraquinones. OMC inhibited carbohydrate metabolizing enzymes in vitro, with IC 50 values of 38.49 μg/ml (α-glucosidase) and 3.4 mg/ml (α-amylase). When orally administered Wistar rats, OMC was demonstrated to inhibit the increase of blood sugar levels in vivo.
Nocardiopsis SCA21 was isolated from marine sediment in India by Siddharth and Rai (2019). Two bioactive compounds were first purified and identified as 4-bromophenol, a bromophenol derivative (6), and Bis (2-ethylhexyl) phthalate, a phthalate ester (7). These two compounds exhibited strong enzyme inhibitory activities against α-glucosidase, with IC 50 values of 94.61 μg/ml (compound 6) and 202.33 μg/ml (compound 7). Compound 6 could also could inhibit the activity of α-amylase with an IC 50 value of 103.23 μg/ml, whereas compound 7 was less active against α-amylase with an IC 50 value >250 μg/ml. Mawlankar et al. (2020) reported a bioactive substance from the novel marine species Arthrobacter enclensis. The purified compound was found to be a α-glucosidase inhibitor, and its IC 50 value was 500 ± 0.142 μg/ ml. It was identified as a C7N aminocyclitol, that displayed high similarity to acarbose, but the structure has not been completely solved. Researchers have also demonstrated that its encoding gene is similar to the biosynthetic gene cluster encoding acarbose (7%). Indupalli et al. (2018) first reported an unusual actinobacterium Saccharomonas oceani VJDS-3 isolated from the mangrove forest in India. The ethyl methoxycinnamate (ethyl (E)-3-4-methoxyphenyl) acrylate (8) was purified. It exhibited significant inhibitory activity against α-glucosidase at 20 μg/ml with an IC 50 value of 66.8 ± 1.2 μg/ml and moderate to weak inhibitory activity against α-amylase at 40 μg/ml with an IC 50 value of 11.5 ± 0.5 μg/ml. Saini and Gangwar (2022) obtained an endophytic actinomycete strain derived from Aegle marmelos. The ethyl acetate extract was assayed to have IC 50 values of 1950.71 ± 0.11 μg/ml against α-amylase, and 391.38 ± 0.09 μg/ml against α-glucosidase. The results indicated that the crude extract has the potential to reduce postprandial blood glucose. However, further studies are needed to identify the strain and the active substance.

Other bacteria
In addition to actinomycetes, other bacteria were reported to produce glycosidase inhibitors, such as Bacillus sp. (nojirimycin; Iida et al., 1987), Vibrio sp. (El-Hady et al., 2017), and lactic acid bacteria (Nurhayati et al., 2017). Scientists have discovered many glycosidase inhibitors from Bacillus sp., Lactobacillus sp., and other bacteria in the past 5 years.
Lactobacillus sp. is another important source of glycosidase inhibitors. Frediansyah et al. (2019) found that all three cell extracts and cell-free supernatants of Lactobacillus pentosus strains isolated from Muntingia calabura L. have inhibitory activity against α-glucosidase and α-amylase in vitro. Moreover, compared to that in the cell-free supernatant group, the cell extracts exhibited higher inhibition of α-glucosidase. However, it seemed to have the opposite effect on the inhibition of α-amylase. In addition, Gulnaz et al. (2021) reported that an extract of Lactobacillus sakei Probio65 and Lactobacillus plantarum Probio-093 has inhibitory activity against α-glucosidase and α-amylase. These two Lactobacillus strains could change the intestinal microbiota diversity in high-fat dietinduced diabetic mice, and Probio-093 had a more significant effect on the intestinal microbiota. Together, this showed that Lactobacillus comprises potential candidates to treat type 2 diabetes. Chi et al. (2020) screened five microbial strains in Vietnam with glycosidase inhibitory effects. The strains were identified as and named Enterobacter cloacae TD-V20, Bacillus sp. TD-V21, Bacillus sp. TD-V24, Streptomyces sp. TD-X13, and Streptomyces sp. TD-X10. All extracts of the five strains exhibited inhibitory activities against α-glucosidase, and Streptomyces sp. TD-X10 showed stronger inhibitory activity against α-amylase. Eleven endophytic bacterial strains from Annona muricata were found to inhibit α-amylase activity. One strain (DS21) exhibited the highest activity with a 72.22% inhibition rate. The strain was a gramnegative bacterium, and needed to be further identified (Pujiyanto et al., 2018).

Inhibitors of glycosidases from fungus
Saito reported an α-amylase inhibitor produced by the fungus Cladosporium herbarum F-28, in contrast to the traditional Frontiers in Microbiology 06 frontiersin.org opinions that only bacteria can produce glycosidase inhibitors, and this inhibitor was found to have high specificity for mammalian amylase (Saito, 1982). Further, glycosidase inhibitory activities were tested using Penicillium (Kwon et al., 2000), the endophytic fungus Stemphylium globuliferum from Trigonella foenum-graceum (Pavithra et al., 2014), Aspergillus awamori isolated from Acacia nilotica (proteinaceous α-glycosidase inhibitor; Singh and Kaur, 2016;Singh et al., 2021), and other fungi. Compared to bacteria, more fungal resources have been found in the past 5 years, including Aspergillus, Penicillium, Mycosphaerella, Alternaria and mushrooms (macrofungi), which make many novel compounds with high inhibitory activity against glycosidases (Table 2).
Frontiers in Microbiology 07 frontiersin.org The Aspergillus egypticus HT166S (endophytic fungus) was isolated from the plant Helianthus tuberosus by Ruzieva et al. (2020). Methanol extracts were found to possess α-amylase inhibitory activity with 75.4% inhibition rate. The methanol component contained flavonoids, terpenoids, anthraquinones, tannins and other bioactive compounds, and flavonoids had the highest activity.
An edible oyster mushroom was also collected from India, and its inhibitory activity against α-amylase was studied by Tamboli et al. (2018). Different extracts were prepared, and methanol, acetone, and chloroform extracts showed inhibitory activities against α-amylase, with IC 50 values of 383, 224, and 1.71 μg/ml, respectively. Further studies on its active composition showed that flavonoids in the acetone extract and glycoproteins in the chloroform extract could be the active components.  tested extracts from two edible macrofungi (Dacryopinax spathularia and Schizophyllumcommune), for their inhibitory activities against α-amylase. The extract of D. spathularia showed an α-amylase inhibition rate of 38.24% at 1000 μg/ml, and the extract of S. commune had a rate of 48.19%.
Frontiers in Microbiology 08 frontiersin.org Stojkovic et al. (2019) investigated the in vitro anti-diabetic properties of six edible and medicinal mushroom species, Agaricus blazei Murrill, Coprinus comatus, Cordyceps militaris, Inonotus obliquus, Morchella conica and Phellinus linteus. The methanol extract of C. comatus showed the highest inhibitory activity against α-amylase with an IC 50 value of 714.45 μg/ml. All of the six tested macrofungi had inhibitory effects against α-glucosidase, and the strongest inhibitory effect was found with I. obliquus, with an IC 50 value of 220.31 μg/ml, which was the most potent strain.

Other fungi
A variety of other fungi, especially endophytic fungi, were also discovered for their inhibitory activities against glycosidases in recent years. Nigronapthaphenyl (34), a new compound extracted from the endophytic fungus Nigrospora sphaerica derived from the mangrove Bruguiera gymnorrhyza was reported by Ukwatta et al. (2019). The new substance displayed strong inhibitory activity against α-glucosidase (IC 50 value of 6.9 ± 0.5 μM). Sharma et al. (2021) isolated the endophytic fungus Schizophyllum commune Fr. from Aloe vera and its extract showed more than 90% inhibitory activity against α-glucosidase. Treatment of STZ-induced diabetic rats with the fungus extract reduced blood glucose levels. Phenols and terpenoids were identified in the ethyl acetate extract, which could be the active ingredients. Saravanakumar et al. (2021) surveyed the glycosidase inhibitory activities of the endophyte Diaporthe eres (SPEF004) derived from the Ligustrum obtusifolium leaf. The ethyl acetate extract of D. eres displayed α-glucosidase inhibitory activity of 13.28 ± 0.94% and α-amylase inhibitory activity of 41.11 ± 1.52%. An endophytic fungal Colletotrichum species derived from Salacia macrosperma was detected by Roopa et al. (2022). The fungal extract showed inhibitory effects against α-glucosidase and α-amylase with IC 50 values of 124.62 and 106.11 μg/ml, respectively.
The marine fungus Talaromyces indigoticus FS688 was studied by Li et al. (2021), and many bioactive compounds were isolated. The compound eurothiocins C (35) had stronger α-glucosidase inhibitory activity (IC 50 value of 5.4 μM), but eurothiocins F (36) and eurothiocins G (37) displayed lower activities with IC 50 values of 33.6 μM and 72.1 μM, respectively. Alcantara et al. (2019) reported the anti-diabetic effects of various extracts of Fuscoporia torulosa MFSLP-12. A methanol extract of this fungus had significant α-glucosidase inhibitory activity, with an inhibition rate of 56%, and α-amylase inhibitory activity with inhibition rate of 38%. The IC 50 value was about 5-fold and 9-fold higher for α-glucosidase and α-amylase than the control drug acarbose.

Pharmacological therapy
Reducing postprandial hyperglycemia is the most important treatment for diabetes. The researchers realized that the inhibition of glucosidase with inhibitors could regulate the absorption of carbohydrates and prevent postprandial hyperglycemia (Asano, 2003). Those inhibitors could cause the release of GLP-1 and reduce glycated hemoglobin levels (Kumar and Sinha, 2012). Glucosidase inhibitors, such as acarbose, voglibose, and miglitol, are important first-line agents for type 2 diabetes patients (Hossain and Pervin, 2018;Dhatariya, 2019). Also, these drugs can be used as second-line agents in combination with metformin, which could reduce the dosage of metformin and improve safety (Clissold and Edwards, 1988;Chan et al., 2018).
Acarbose (38), isolated from Actinoplanes sp. SE50, was the first commercialized glucosidase inhibitor launched in 1990 (Schmidt et al., 1977;Joshi et al., 2014). It is one of the most common glycosidase inhibitors, and also the most widely studied one. Acarbose inhibits many glycosidases, such as α-amylase, maltase, and glucoamylase, which could reduce the hydrolysis of starch in intestine (Liu and Ma, 2017;Li et al., 2022). Voglibose (39), isolated from Streptomyces hygroscopicies limonons, was discovered in 1981 (Japan), and marketed for clinical use since 1994 (Horii et al., 1986;Saito et al., 1998). It is a more tolerated and potent inhibitor of α-glycosidase than acarbose with fewer side effects and higher activities (Dabhi et al., 2013). Miglitol (40) was developed by Bayer and first marketed in 1998 (Kumar and Sinha, 2012). It is a derivative of 1-desoxynojirimycin (DNJ). DNJ can be isolated from S. lavendulae or other strains, and then chemically synthesized to form miglitol (Sels et al., 1999). Miglitol is completely absorbed by the small intestine with high bioavailability, whereas acarbose and voglibose are poorly absorbed with low bioavailability (Akmal and Wadhwa, 2022; Figure 3).

Dietary supplements
The presence of glycosidase inhibitors in the diet can inhibit the activity of human glycosidase and reduce the absorption of dietary carbohydrates . In addition, there has been increasing concern about the possibility of using dietary supplements to prevent diabetes. For example, Salacia reticulata is used as a diabetic supplement in Japan (Asano, 2003;Zou et al., 2019).

Obesity
Diet control is an important way to control obesity. Glycosidase are responsible for carbohydrate digestion, and increase postprandial blood sugar levels. Glycosidase inhibitors are potential compounds that can be used in weight loss. They inhibit glucosidase, delay the absorption of carbohydrates, and reduce people's postprandial blood sugar levels and insulin responses to dietary carbohydrates (Peddio et al., 2022).
Also, another important factor for obesity is the abnormal differentiation or adipocytes dysfunction. Li et al. illustrated that DNJ, an α-glucosidase inhibitor, can inhibit adipogenesis during the differentiation of white preadipocytes, providing a new approach to explain the beneficial effects of α-glucosidase inhibitor on obesity .

Antiviral treatment
N-nonyl-deoxynojirimycin (NN-DNJ, α-glucosidase inhibitor derivative) is a potential antiviral drug. Block et al. reported that NN-DNJ induces misfolding of the hepatitis B virus envelope glycoproteins and further prevents virus formation (Block et al., 1998). Moreover, Zitzmann et al. demonstrated that NN-DNJ could prevent the formation and secretion of bovine viral diarrhea virus, a model for human hepatitis C virus (Zitzmann et al., 1999;Whitby et al., 2004). Also, N-butyl-deoxynojirimycin (α-glucosidase inhibitor) and its derivatives show significant antiviral against Ebola virus in vitro. It inhibits assembly and secretion of virus particle (Chang et al., 2013;Yuan, 2015).

Lysosomal storage diseases
The lysosomal storage diseases are a group of inherited diseases that lead to metabolic disorders of the lysosomes. The diseases mainly include Fabry disease, Gaucher disease, Niemann-Pick disease and so on (Khanna et al., 2010;Mechtler et al., 2012). Giraldo et al. reported the treatment of type 1 Gaucher disease with N-butyl-deoxynojirimycin (α-glucosidase inhibitor) over 12 years. Eventually 80% of patients achieved the treatment goals, with stable levels of hematologic counts and volumes of the liver and spleen (Giraldo et al., 2018).

Conclusion and further research
In recent years, the incidence of type 2 diabetes has been growing rapidly. Glucose, which is hydrolyzed by glycosidases, is absorbed into the blood, and then caused severe postprandial hyperglycemia. So glycosidase is an important therapeutic target for diabetes (Usman et al., 2019). However, marketed inhibitors of glycosidase have many side effects. Therefore, novel glycosidase inhibitors that are safer, more effective, and more cost-effective are needed.
Glycosidases are produced by microbes, animals and plants (Aslan et al., 2018;Demir et al., 2018). However, inhibitors of glycosidase are mainly derived from microbes. In the most recent 5 years, increasing research on new microorganisms producing inhibitors of glycosidase has been reported. Compared to general microorganisms, most new microorganisms are extremophiles, which is reflected by the number of papers on new microorganisms. This new focus on extremophiles expands the scope of the search glycosidase inhibitors. Five reports introduced nine new compounds with inhibitory activities against glycosidases in recent 5 years. They are compound 14 isolated from the marine fungus Aspergillus terreus OUCMDZ-2739 by Sun et al.; five new compounds 15-19 from a marine fungus Penicillium sp. by Guo et al.; novel xanthone (21) isolated from endophytic Penicillium canescens strain by Malik et al.;asperchalasine I (30) isolated from the mangrove fungus Mycosphaerella sp. by Qiu et al.;and Nigronapthaphenyl (34) extracted from the endophytic fungus Nigrospora sphaerica by Ukwatta et al. All of them come from marine and endophytic fungus. It would be very useful if a database for microorganisms and its glycosidase inhibitors was established.
In addition to applications in the treatment of diabetes and obesity, several applications for glycosidase inhibitors have been reported but have not yet been industrially developed. We look forward to seeing the use of these inhibitors expand.
Author contributions XW, JL, and JS reviewed conceptualization and wrote the manuscript. JB, KW, JL, and ZY collected the data from previous researches and prepared the figures and tables. HO designed of the work and reviewed critically for important intellectual content. LS designed and supervised the paper. All authors contributed to the article and approved the submitted version.