Department of Pharmacology, University College of Pharmaceutical Sciences, Kakatiya University, Hanumakonda, Telangana – 506001, India
Globally, diabetes mellitus represents one of the most challenging chronic metabolic disorders of the 21st century, with type 2 diabetes mellitus (T2DM) accounting for the overwhelming majority of cases. Postprandial hyperglycemia—the acute elevation of blood glucose following carbohydrate ingestion—is a central pathophysiological feature of T2DM and a known risk factor for macro- and microvascular complications. Therapeutic inhibition of intestinal alpha-amylase, the enzyme catalysing the hydrolysis of dietary starch into absorbable sugars, is a clinically recognised strategy for dampening postprandial glucose surges. The current study investigated the antidiabetic potential of methanolic parotid gland extract (PGE) obtained from the Indian toad, Duttaphrynus melanostictus, using a standardized 3,5-dinitrosalicylic acid (DNSA) colorimetric alpha-amylase inhibition assay. Working concentrations of 25, 50, and 100 ?g/mL were prepared and tested in triplicate against porcine pancreatic alpha-amylase, with Acarbose serving as the positive control. The PGE exhibited concentration-dependent inhibition of 26.15%, 46.15%, and 69.23% at 25, 50, and 100 ?g/mL, respectively. The IC?? of the extract was determined to be approximately 72.4 ?g/mL by linear regression, compared to48.6 ?g/mL for Acarbose. Despite exhibiting moderate potency relative to the reference drug, the PGE demonstrated statistically significant inhibition (p<0.05) at all concentrations tested. These findings provide compelling evidence that the parotid secretions of D. melanostictus contain bioactive compounds capable of interfering with carbohydrate digestion, highlighting the pharmacological relevance of amphibian-derived products as candidate scaffolds for novel antidiabetic therapeutics..
Diabetes mellitus (DM) is a complex, multifactorial endocrine disorder characterized by chronic hyperglycemia resulting from absolute or relative deficiency of insulin secretion, impaired insulin sensitivity at target tissues, or a combination of both mechanisms. The condition is globally recognized as a major public health crisis of escalating proportions. According to the International Diabetes Federation (IDF) Diabetes Atlas (10th Edition, 2021), an estimated 537 million adults worldwide were living with diabetes as of 2021, a figure projected to rise to 783 million by 2045 if current trends persist. Type 2 diabetes mellitus (T2DM) constitutes approximately 90–95% of all diabetic cases and is closely associated with overnutrition, physical inactivity, abdominal obesity, and a polygenic predisposition to insulin resistance. A defining and therapeutically important feature of early-stage T2DM is postprandial hyperglycaemia—the transient yet significantly elevated blood glucose that follows carbohydrate ingestion. This phenomenon results from the rapid enzymatic breakdown of dietary starch by intestinal and salivary alpha-amylase into maltose and oligosaccharides, which are subsequently hydrolysed to glucose by alpha-glucosidases at the intestinal brush border and rapidly absorbed into systemic circulation. Repeated and sustained postprandial glucose spikes impose oxidative stress on pancreatic beta cells and endothelial tissues, contributing to the pathogenesis of diabetic nephropathy, retinopathy, neuropathy, and accelerated cardiovascular disease. One well-established pharmacological strategy to attenuate postprandial glycemic excursions involves inhibiting the activity of alpha-amylase at the intestinal level. Acarbose, a pseudotetrasaccharide derived from Actinoplanes is the prototype and most clinically used alpha-amylase/alpha-glucosidase inhibitor. While effective in reducing postprandial glucose peaks, Acarbose is frequently associated with dose-dependent gastrointestinal adverse events, including flatulence, abdominal distension, osmotic diarrhoea, and bloating, which arise from colonic fermentation of undigested carbohydrates. These limitations have sustained considerable scientific interest in identifying safer, naturally occurring inhibitors with comparable or complementary mechanisms of action.
Amphibians have long been recognized as a remarkably productive source of structurally novel and pharmacologically diverse bioactive molecules. Their evolutionary pressure from microbial pathogens and predators has driven the elaboration of chemically complex skin and glandular secretions that serve as defense mechanisms. Among South and Southeast Asian anurans, Duttaphrynus melanostictus—commonly referred to as the Indian toad or Asian common toad—is one of the most widespread and pharmacologically studied species. This toad is readily identified by its prominent parotid glands, which are large, kidney-shaped exocrine structures situated dorsolateral behind the tympanum. These glands elaborate a heterogeneous mixture of bioactive substances including bufadienolides (Bufalin, Cinobufagin, Marinobufagin), indole alkaloids (bufotenine), biogenic amines (adrenaline, noradrenaline, dopamine), steroids, and various bioactive peptides. The pharmacological repertoire attributed to these compounds spans anticancer, cardiotonic, analgesic, antimicrobial, and enzyme-inhibitory activities. A prior investigation by Neerati (2015) documented preliminary antidiabetic activity from crude parotoid gland secretions of the common Indian toad, although a systematic in vitro enzyme inhibition study using standardised methodology was not performed. Given the compositional richness of D. melanostictus parotid secretions and the documented enzyme-modulatory properties of bufadienolides and steroid-type compounds in the existing literature, the present study was designed to rigorously evaluate the in vitro alpha-amylase inhibitory activity of methanolic parotid gland extract (PGE) of D. melanostictus using the 3,5-dinitrosalicylic acid (DNSA) colorimetric assay, with concurrent determination of IC?? values and statistical comparison against the established inhibitor Acarbose.
MATERIALS AND METHODS
2.1 Collection and Identification of Animal Specimens
Adult specimens of Duttaphrynus melanostictus were collected from peri-urban localities and agricultural peripheries surrounding Hanumakonda district, Telangana, during the active season (March to October). Taxonomic identification was confirmed using standard morphological criteria and established amphibian identification keys. All animal procedures were conducted in strict compliance with the guidelines issued by the Committee for the Purpose of Control and Supervision of Experiments on Animals (CPCSEA), New Delhi, and with the approval of the Institutional Animal Ethics Committee (IAEC). Toads were maintained in ventilated, humidified containers and handled exclusively with nitrile gloves and protective eyewear to prevent accidental exposure to glandular secretions.
2.2 Preparation of Parotid Gland Extract
The parotid glands were carefully dissected from the dorsolateral regions of toads anaesthetised with diethyl ether under sterile surgical conditions. Excised glands were washed multiple times with sterile distilled water to eliminate surface contaminants and extraneous biological material, then preserved at −20°C until further processing. For methanolic extraction, glandular tissue was homogenised in analytical-grade methanol at a ratio of 1:10 (w/v) using a pre-chilled mortar and pestle. The homogenate was centrifuged at 5000 rpm for 15 minutes at 4°C; the resultant supernatant was collected and filtered through Whatman No. 1 filter paper. The filtrate was concentrated to a semi-solid residue under reduced pressure using a rotary evaporator (Heidolph, Germany), and the dried extract was reconstituted in 0.02 M phosphate buffer (pH 6.9) to prepare working concentrations of 25, 50, and 100 μg/mL for bioassay.
2.3 Chemicals and Reagents
All chemicals were procured as analytical-grade reagents. Porcine pancreatic alpha-amylase (Sigma-Aldrich, St. Louis, USA), soluble potato starch (HiMedia Laboratories, India), 3,5-dinitrosalicylic acid (DNSA; Sigma-Aldrich), sodium potassium tartrate, sodium hydroxide, Acarbose (reference standard; Sigma-Aldrich), and 0.02 M phosphate buffer (pH 6.9) were utilised throughout the study. All solutions were freshly prepared in double-distilled water on each day of assay.
2.4 In Vitro Alpha-Amylase Inhibition Assay (DNSA Method)
The alpha-amylase inhibitory activity of PGE was evaluated using a DNSA colorimetric method adapted from the procedure originally described by Bernfeld (1955) and subsequently modified for microplate compatibility. Briefly, 0.5 mL of each concentration of test extract (25, 50, or 100 μg/mL) was dispensed into separate labelled test tubes and mixed with 0.5 mL of porcine pancreatic alpha-amylase solution (1 mg/mL, prepared in phosphate buffer, pH 6.9). The enzyme–inhibitor mixtures were pre-incubated at 37°C in a water bath for 10 minutes to allow sufficient inhibitor binding prior to substrate addition.
Following pre-incubation, 0.5 mL of 1% (w/v) soluble starch solution (prepared in phosphate buffer) was added as the substrate to initiate the enzymatic reaction. The reaction was allowed to proceed at 37°C for an additional 10 minutes. Enzymatic activity, was halted by the addition of 1.0 mL of freshly prepared DNSA colour reagent (containing DNSA and sodium potassium tartrate in 0.4 M NaOH). All tubes were subsequently transferred to a boiling water bath (100°C) for precisely 5 minutes to develop the characteristic red-brown colour proportional to reducing sugar liberated. Following cooling to room temperature, each tube was diluted with 8 mL of distilled water, and absorbance was recorded at 540 nm using a UV-Visible spectrophotometer (Shimadzu UV-1800, Japan).A negative control (consisting of enzyme, starch, and buffer without extract) and a positive control (Acarbose at equivalent concentrations) were assayed concurrently. Percentage alpha-amylase inhibition was determined using the formula:
% Inhibition = [(Ac – As) / Ac] × 100
Where Ac represents the absorbance of the negative control and As the absorbance of the test sample at 540 nm. The IC?? (concentration producing 50% inhibition) for both PGE and Acarbose was determined by linear regression analysis of percentage inhibition plotted against the logarithm of concentration.
2.5 Statistical Analysis
All experimental data are expressed as Mean ± Standard Error of Mean (SEM; n = 3). One-way analysis of variance (ANOVA) followed by Tukey's multiple comparison post hoc test was used to evaluate statistical differences between treatment groups. A p-value of <0.05 was considered statistically significant for all comparisons. Data analysis and graphical representation were carried out using GraphPad Prism version 9.0 (GraphPad Software Inc., San Diego, USA).
RESULTS AND DISCUSSION
The methanolic parotid gland extract of Duttaphrynus melanostictus exhibited a clear, statistically significant, and concentration-dependent inhibition of porcine pancreatic alpha-amylase activity across all three concentrations evaluated. The detailed quantitative results of the alpha-amylase inhibition assay are presented in Table 1 below.
Table 1: Alpha-amylase inhibition assay observations (Mean ± SEM, n = 3). PGE = Parotid Gland Extract.
|
S. No |
Sample / Inhibitor |
Concentration (μg/mL) |
Absorbance of Control (A???) |
Absorbance of Sample (A???) |
% Inhibition |
|
1 |
Negative Control |
0 |
0.650 |
0.650 |
– |
|
2 |
PGE |
25 |
0.650 |
0.480 |
26.15 |
|
3 |
PGE |
50 |
0.650 |
0.350 |
46.15 |
|
4 |
PGE |
100 |
0.650 |
0.200 |
69.23 |
|
5 |
Acarbose (Standard) |
25 |
0.650 |
0.400 |
38.46 |
|
6 |
Acarbose (Standard) |
50 |
0.650 |
0.250 |
61.54 |
|
7 |
Acarbose (Standard) |
100 |
0.650 |
0.150 |
76.92 |
At the lowest tested concentration of 25 μg/mL, the PGE inhibited alpha-amylase activity by 26.15%. This inhibitory effect progressively intensified with increasing concentration, reaching 46.15% at 50 μg/mL and 69.23% at 100 μg/mL, clearly demonstrating a dose–response relationship. In parallel, Acarbose produced inhibitions of 38.46%, 61.54%, and 76.92% at the same concentration points, indicating a higher potency on a mass-per-volume basis. The IC?? values derived by linear regression are summarized in Table 2.
Table 2: Comparative IC?? values of PGE and Acarbose.
|
Compound |
IC?? (μg/mL) |
|
PGE |
∼72.4 |
|
Acarbose |
∼48.6 |
The IC?? of PGE (approximately 72.4 μg/mL) was approximately 1.5-fold higher than that of Acarbose (approximately 48.6 μg/mL), indicating that the crude extract requires a greater concentration to achieve equivalent inhibition. However, it is important to contextualize this comparison: Acarbose is a highly purified, pharmaceutically optimised competitive inhibitor of microbial origin, while the PGE tested in this study represents a complex, un-fractionated crude mixture that inevitably contains pharmacological inert or even antagonistic components that dilute the overall inhibitory potency. The fact that a crude, unoptimized biological extract achieves an IC?? within the same order of magnitude as a clinically approved drug is pharmacologically noteworthy and warranting of further investigation.
The concentration-dependent nature of inhibition strongly argues for the participation of specific bioactive molecules in the PGE that engage the active or allosteric sites of alpha-amylase in a saturable, dose-responsive fashion, consistent with enzyme inhibition kinetics. Among the known constituents of D. melanostictus parotid secretions, bufadienolides such as Bufalin and Cinobufagin have drawn particular attention. These steroidal compounds possess structural features—polyhydroxylated steroid cores with lactone rings—that are theoretically capable of hydrogen bonding and hydrophobic interactions with the catalytic residues of alpha-amylase (Asp197, Glu233, Asp300 in porcine pancreatic amylase). Additionally, biogenic amines such as adrenaline and noradrenaline, and indole alkaloids like bufotenine, may contribute to enzyme inhibition through complementary mechanisms. The precise identification of the inhibitory constituent(s) awaits bioassay-guided fractionation, isolation, and structural characterization.The results obtained in this study are concordant with the broader pharmacological literature establishing natural product-based alpha-amylase inhibitors as viable candidates for postprandial hyperglycaemia management. Numerous plant-derived polyphenols, flavonoids, terpenoids, and alkaloids have been documented to inhibit alpha-amylase in vitro, and several have demonstrated efficacy in preclinical in vivo models. Marine-derived compounds and microbial metabolites have also contributed to this drug discovery pipeline. The current findings extend this therapeutic paradigm to vertebrate amphibian glandular products, a source category that has been comparatively underutilized in the context of antidiabetic drug discovery. It is pertinent to note that all assays were performed using porcine pancreatic alpha-amylase, which shares high structural and functional homology with human pancreatic and salivary alpha-amylases, thereby supporting the translational relevance of the in vitro inhibitory data. All results achieved statistical significance (p<0.05) relative to the negative control at each concentration tested, confirming that the observed inhibition was not due to random variation or assay artefact.Several important limitations of the present study must be acknowledged. First, only the crude methanolic extract was investigated, without phytochemical or metabolomic characterisation of its constituents. Second, the study was conducted entirely in vitro; extrapolation to in vivo antidiabetic efficacy requires validation in appropriate animal models. Third, toxicological data specific to the parotid gland extract were not obtained, though preliminary safety data for toad skin extract from the same species (LD?? of 2000 mg/kg by oral route in Wistar rats) are available from related institutional studies and provide a useful frame of reference. Future research should prioritise: (i) bioassay-guided isolation and structural elucidation of active constituents; (ii) molecular docking and kinetic studies to define the mechanism of inhibition; (iii) acute and sub-chronic toxicity profiling of PGE; and (iv) in vivo antidiabetic evaluation in streptozotocin-induced diabetic rodent models, inclusive of oral glucose tolerance tests and postprandial glucose monitoring.
CONCLUSION
The present investigation conclusively demonstrates that the methanolic parotid gland extract of Duttaphrynus melanostictus possesses significant, concentration-dependent in vitro inhibitory activity against porcine pancreatic alpha-amylase, as quantified by the DNSA colorimetric method. Inhibitory activity ranged from 26.15% at 25 μg/mL to 69.23% at 100 μg/mL, with an IC?? of approximately 72.4 μg/mL compared to 48.6 μg/mL for the reference inhibitor Acarbose. Although the crude extract displayed moderately lower potency than the purified standard, its statistically significant inhibitory effect across all concentrations tested, and its concentration-dependent pattern, substantiate the presence of genuine bioactive constituents capable of modulating carbohydrate-metabolizing enzyme activity. These findings establish a pharmacological basis for further investigation of toad parotid-derived compounds as potential leads in the development of novel, naturally sourced antidiabetic agents.
ACKNOWLEDGEMENTS
The authors extend their sincere gratitude to the Department of Pharmacology, University College of Pharmaceutical Sciences, Kakatiya University, Hanumakonda, Telangana, for providing the necessary laboratory facilities, instrumentation, and institutional support required to conduct this research. Special thanks are due to the technical staff of the Department for their assistance during animal handling and biochemical assay procedures. The authors also acknowledge Dr. Sangeethkumar Munigadapa, Assistant Professor, Department of Pharmacology, for his valuable guidance and mentorship throughout the study.
Conflict of Interest: The authors declare that this research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
REFERENCES
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10.5281/zenodo.20095605
