New Sweet-Tasting C21-Pregnane Glycosides from Pericarps of Myriopteron extensum

A review on natural sweeteners, sweet taste modulators and bitter masking compounds: structure-activity strategies for the discovery of novel taste molecules

Growing demand for the tasty and healthy food has driven the development of low-calorie sweeteners, sweet taste modulators, and bitter masking compounds originated from natural sources. With the discovery of human taste receptors, increasing numbers of sweet taste modulators have been identified through human taste response and molecular docking techniques. However, the discovery of novel taste-active molecules in nature can be accelerated by using advanced spectrometry technologies based on structure-activity relationships (SARs). SARs explain why structurally similar compounds can elicit similar taste qualities. Given the characterization of structural information from reported data, strategies employing SAR techniques to find structurally similar compounds become an innovative approach to expand knowledge of sweeteners. This review aims to summarize the structural patterns of known natural non-nutritive sweeteners, sweet taste enhancers, and bitter masking compounds. Innovative SAR-based approaches to explore sweetener derivatives are also discussed. Most sweet-tasting flavonoids belong to either the flavanonols or the dihydrochalcones and known bitter masking molecules are flavanones. Based on SAR findings that structural similarities are related to the sensory properties, innovative methodologies described in this paper can be applied to screen and discover the derivatives of taste-active compounds or potential taste modulators.

Sensory-guided isolation and identification of new sweet-tasting dammarane-type saponins from Jiaogulan (Gynostemma pentaphyllum) herbal tea

The purpose of this study was to elucidate the chemical basis for the sweet property produced by Gynostemma pentaphyllum and find new natural high-potency (HP) sweeteners. Sixteen new compounds (gypenosides YN 1-16) were obtained by sensory-guided isolation and identification, in which fifteen of them were sweet-tasting constituents with sweetness intensities 10-100 times higher than that of sucrose evaluated by human sensory panel test. Their structures were established by 1D and 2D nuclear magnetic resonance spectra, mass spectroscopy, infrared spectroscopy, UV-visible spectroscopy, and chemical method. Gypenoside YN 4 was the sweetest compound with a concentration of 15.504 ± 1.343 mg/kg, while gypenoside YN 12 has the highest concentration (1397.674 ± 12.948 mg/kg), as shown by ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Structure-activity relationship analysis implied that the compounds' sweetness intensity was associated with side-chain substitutions at C-20 or the number of glucosyl groups at C-3. These new plant-derived natural products may be potential natural sweeteners.

Pregnane glycosides from Adonis amurensis and their bioactivity

Seven undescribed pregnane glycosides named amurensides A-G and two known aglycones were isolated from the whole herb of Adonis amurensis Regel & Radde. Their structures were established based on 1D and 2D NMR spectroscopic analyses, high-resolution mass spectrometry, and acid hydrolysis. The cytotoxicity of all compounds against three tumor cell lines (HepG2, Caco-2, and A549) were evaluated. Among them, amurensides A-C and E showed moderate inhibitory effects on the growth of HepG2 cells, with IC₅₀ values ranging from 15.6 to 48.7 μM (sorafenib, 7.5 ± 1.9 μM). Amurensides A、D and F displayed inhibitory effects on the growth of A549 cells with IC₅₀ values of 18.8 ± 1.2, 12.4 ± 0.6, and 30.4 ± 0.1 μM (cis-platinum, 6.1 ± 0.1 μM), respectively.

Structure-Dependent Activity of Plant-Derived Sweeteners

Human sensation for sweet tastes and the thus resulting over-consumption of sugar in recent decades has led to an increasing number of people suffering from caries, diabetes, and obesity. Therefore, a demand for sugar substitutes has arisen, which increasingly has turned towards natural sweeteners over the last 20 years. In the same period, thanks to advances in bioinformatics and structural biology, understanding of the sweet taste receptor and its different binding sites has made significant progress, thus explaining the various chemical structures found for sweet tasting molecules. The present review summarizes the data on natural sweeteners and their most important (semi-synthetic) derivatives until the end of 2019 and discusses their structure–activity relationships, with an emphasis on small-molecule high-intensity sweeteners.

De novo transcriptome analysis deciphered polyoxypregnane glycoside biosynthesis pathway in Gymnema sylvestre

Gymnema sylvestre is an important medicinal plant containing antidiabetic activity. Through de novo transcriptomic study, the pathways of polyoxypregnane glycosides were explored and candidate genes of these pathways were identified in G. sylvestre. High-quality raw reads were assembled into transcripts which resulted in 193,615 unigenes. These unigenes further decoded 58,274 coding DNA sequences (CDSs). Functional annotation of predicted CDSs was carried out using the protein databases, i.e., NCBI's non-redundant, Uniprot and Pfam. Eukaryotic orthologous group (KOG) classification and transcription factor analysis has revealed most CDS-enriched categories as "Signal transduction mechanism" and "Basic Helix loop helix" (bHLH) transcription factor family, respectively. A total of 16,569 CDSs were assigned minimum one Gene Ontology (GO) term. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis disclosed 235 CDSs which represented total 27 genes of pregnane glycoside pathways and 19 CDSs represented 10 important enzymes of polyoxypregnane glycoside biosynthesis, i.e., sterol 24-C-methyltransferase, cycloeucalenol cycloisomerase, Δ¹⁴-sterol reductase, C-8,7 sterol isomerase, sterol methyltransferase 2, C-5 sterol desaturase, sterol Δ⁷ reductase, Δ²⁴ sterol reductase, 3β-hydroxysteroid dehydrogenase and progesterone 5β reductase (5βPOR). This transcriptome analysis provided an important resource for future functional genomic studies in G. sylvestre.

New Sweet-Tasting C21 Pregnane Glycosides from the Roots of Myriopteron extensum

To investigate the sweet-tasting components in the roots of Myriopteron extensum, the phytochemical study of its roots was conducted, which led to the discovery of 12 new C21 pregnane glycosides (extensumside M-X, 1-12) and two known ones (extensumside C and extensumside E, 13-14). Their chemical structure elucidation was accomplished by means of spectroscopic methods: IR, UV, ESI-MS, and NMR (¹H NMR, ¹³C NMR, HSQC, ¹H-¹H COSY, HMBC, HSQC-TOCSY, and ROESY), as well as the chemical evidence. Sensory analysis of these compounds revealed that nine of them (1, 3, 4, 5, 6, 7, 8, 13, and 14) are highly sweet-tasting compounds. Their sweetness intensities are 25 to 400 times greater than that of sucrose. Analysis of the structure-activity relationship (SAR) indicated that the sweet intensities of the isolated compounds are closely related to the aglycone 3β,16α-dihydroxy-pregn-5-en-20-one, the number and type of the monosaccharide in the sugar chain linked to C-3 and C-16 and the position of the mBe group.