Chemical constituents of Impatiens parvisepala and their α-glucosidase inhibition activity

Abstract The first chemical study of the whole Impatiens parvisepala S. X. Yu & Y. T. Hou led to the isolation of a new triterpene saponin, named Iparvisepala-1 (6) along with ten known compounds, which cover three flavonoid glycosides (1-3), one dihydrochalcone glucoside (4), one triterpenoid saponin (5), one triterpene (7) and four miscellaneous compounds (8-11). Their structures were elucidated by MS, NMR spectroscopic analyses as well as by comparisons of spectra data with those of the related published literatures. Additionally, flavonoid glucoside 2 showed impressive effect on α-glucosidase inhibition with the IC50 value of 12.53 ± 0.39 µM, much better than that of the positive control acabose (IC50 = 197.53 ± 2.68 µM). Graphical Abstract


Introduction
Balsaminaceae, an old family contains only two genera Hydrocera and Impatiens with more than 1000 species of the latter genus . Being a huge genus, Impatiens is now distributed in different parts of the world, of which 240 species have been found in China (Hou et al. 2011) and 34 species in Vietnam (Ho 2000) in particular. More than 300 compounds of various groups have been isolated and identified from total 27 Impatiens species so far with a part of which are the material basis of different pharmacological effects (Singh et al. 2017, Szewczyk 2018. For instances, the purified phenolic acid ferulic acid isolated from Impatiens bicolor was proved as a potential trypsin inhibitor (Shahwara et al. 2012), isoquercitrin obtained from I. glandulifera showed antioxidant activities comparable to ascorbic and trolox (Szewczyk et al. 2019), naphthoquinones from I. balsamina significantly exhibited antimicrobial activities (Sakunphueak and Panichayupakaranant 2012), galactolipids derived from the root of I. parviflora were showed as potent inhibitors of hyaluronidase (Grabowska et al. 2016) and the flavonoid (±)-3 0 ,5 0 ,5,7-tetrahydroxy flavanone isolated from I.chapaensis remarkably inhibited a-glucosidase (Linh et al. 2021).
Impatiens parvisepala S. X. Yu & Y. T. Hou has been detected in Guangxi, China as a new species (Hou et al. 2011) and accepted in plants of the world online royal botanic garden Kew (Govaerts et al. 2021). This plant was then found in Tuyen Quang, Vietnam in 2015 for the first time by Hoang Thanh Son (Son et al. 2015). As described, I. parvisepala is perennial shrub with the high up to 50 cm. To the best of our knowledge, there was no previous studies analysing chemical constituents as well as biological activities on I. parvisepala until now. This prompted a chemical investigation, resulting in the isolation and structural elucidation of a new triterpenoid saponin, named Iparvisepala-1 (6), together with ten known compounds from the whole plant of I. parvisepala. Among the known isolates, seven (1, 5 and 7-11) were not previously identified within the Impatiens genus. The results of the literature search showed that, three purified substances (2, 8 and 10) have not been investigated for antidiabetic activity before. Therefore, these three compounds were tested for a-glucosidase inhibitory activity herein to result the noticeable effect of compound 2 against a-glucosidase enzyme (IC 50 ¼ 12.53 ± 0.39 mM).

a-Glucosidase inhibition assay
In the course of our ongoing search for potential a-glucosidase inhibitors from Impatiens species, we now report the a-glucosidase inhibition assay (Liu et al. 2016) of three chosen compounds (2, 8 and 10). The results indicated that flavonoid glucoside 2 showed the most promising activity against a-glucosidase with an IC 50 value of 12.53 ± 0.39 mM, which was significantly better than that observed for the positive control, acabose (IC 50 ¼ 197.53 ± 2.68 mM). In contrast to flavonoid structure, quinone derivative and glycerol (8 and 10) failed to inhibit the tested enzyme (IC 50 > 1000 mM).

Statistical analyses
Statistical calculations were performed using the Microsoft Excel software. Data are expressed as means ± standard deviation (SD), as described in detail in the Supplementary material.

Conclusion
This study is the first to describe phytochemical constituents of I. parvisepala through the isolation and structural elucidation of 11 different compounds. Out of these isolates, compound 6 was elucidated as a new triterpenoid saponin, while seven (1, 5, and 7-11) were the first representatives of the genus Impatiens. In vitro evaluation of the a-glucosidase inhibitory potential was carried out on three isolated metabolites (2, 8 and 10) to result the most suppressor effect of apigenin-7-O-glucoside (2) on the tested enzyme (IC 50 ¼ 12.53 ± 0.39 mM).

Disclosure statement
No potential conflict of interest was reported by the authors.