Ethyl Acetate Fraction of Helianthus tuberosus L. Induces Anti-Diabetic, and Wound-Healing Activities in Insulin-Resistant Human Liver Cancer and Mouse Fibroblast Cells

Traditional, complementary, and integrative medicine are globally accepted alternative methods for the treatment of diabetes mellitus (DM). However, the mechanism of anti-diabetic effects of Helianthus tuberosus L. remains unproven. In the present study, antioxidant and anti-diabetic activity of the tubers of H. tuberosus were studied in detail. Methanolic extracts of H. tuberosus tubers were subjected to solvent fractionation method by increasing the polarity of the solvent using n-hexane, and ethyl acetate. The obtained methanol extracts and its fractions were subjected to free radical scavenging activity (DPPH and ABTS assay) and in vitro enzyme (α-amylase and α-glucosidase) inhibition assay. Moreover, glucose uptake in insulin-resistant HepG2 cell line was analyzed. The preliminary phytochemical analysis confirmed the presence of phenolic and flavonoid compounds in the active fraction. The radical scavenging and in vitro diabetic related enzyme inhibitory activities were found to be dose dependent. The maximum ABTS⁺ and DPPH scavenging activity was documented in ethyl acetate fraction of the H. tuberosus followed by methanol extract, hexane fraction, and methanol fraction. We also found that H. tuberosus showed a less toxicity in mouse fibroblast cells and enhance the glucose uptake in insulin-resistant HepG2 cells. Besides, the ethyl acetate fraction of the H. tuberosus analyzed by UPLC-QTOF-MS-MS and GC/MS revealed the presence of phenolic compounds such as neochlorogenic acid, chlorogenic acid, caffeic acid, 5-O-(4-coumaroyl)-quinic acid, feruloylquinic acid, caffeoylquinic acid, isoxazolidine, salicylic acid β-D-glucoside, dicaffeoylquinic acid isomers, salvianolic acid derivative isomers, and 1,4 dicaffeoylquinic acid etc. Among the identified phytochemicals, six were chosen for molecular docking study to explore their its inhibitory interactions with α-amylase and α-glucosidase. Taken together, the findings of the present study suggested that phytocompounds of EAF were responsible for the significant in vitro antioxidant, wound-healing, and anti-diabetic activities.

Convenient access to pyrrolidin-3-ylphosphonic acids and tetrahydro-2H-pyran-3-ylphosphonates with multiple contiguous stereocenters from nonracemic adducts of a Ni(II)-catalyzed Michael reaction

A new synthetic strategy toward nonracemic phosphoryl-substituted pyrrolidines and tetrahydropyranes with three and five contiguous stereocenters is presented. Readily available β-keto phosphonates react with conjugated nitroolefins in the presence of a chiral Ni(II) complex to give nitro keto phosphonates with two stereocenters with excellent enantioselectivity and moderate to high diastereoselectivity. These products were used for a reductive cyclization leading to pyrrolidin-3-ylphosphonic acid and for reactions with aldehydes yielding tetrahydropyranylphosphonates as individual stereoisomers. These nonracemic heterocycles containing phosphoryl moieties are useful for designing new pharmacologically active compounds.

Functional and structural basis of E. coli enolase inhibition by SF2312: a mimic of the carbanion intermediate

Many years ago, the natural secondary metabolite SF2312, produced by the actinomycete Micromonospora, was reported to display broad spectrum antibacterial properties against both Gram-positive and Gram-negative bacteria. Recent studies have revealed that SF2312, a natural phosphonic acid, functions as a potent inhibitor of human enolase. The mechanism of SF2312 inhibition of bacterial enolase and its role in bacterial growth and reproduction, however, have remained elusive. In this work, we detail a structural analysis of E. coli enolase bound to both SF2312 and its oxidized imide-form. Our studies support a model in which SF2312 acts as an analog of a high energy intermediate formed during the catalytic process. Biochemical, biophysical, computational and kinetic characterization of these compounds confirm that altering features characteristic of a putative carbanion (enolate) intermediate significantly reduces the potency of enzyme inhibition. When SF2312 is combined with fosfomycin in the presence of glucose-6 phosphate, significant synergy is observed. This suggests the two agents could be used as a potent combination, targeting distinct cellular mechanism for the treatment of bacterial infections. Together, our studies rationalize the structure-activity relationships for these phosphonates and validate enolase as a promising target for antibiotic discovery.

The 3S Enantiomer Drives Enolase Inhibitory Activity in SF2312 and Its Analogues

We recently reported that SF2312 ((1,5-dihydroxy-2-oxopyrrolidin-3-yl)phosphonic acid), a phosphonate antibiotic with a previously unknown mode of action, is a potent inhibitor of the glycolytic enzyme, Enolase. SF2312 can only be synthesized as a racemic-diastereomeric mixture. However, co-crystal structures with Enolase 2 (ENO2) have consistently shown that only the (3S,5S)-enantiomer binds to the active site. The acidity of the alpha proton at C-3, which deprotonates under mildly alkaline conditions, results in racemization; thus while the separation of four enantiomeric intermediates was achieved via chiral High Performance Liquid Chromatography (HPLC) of the fully protected intermediate, deprotection inevitably nullified enantiopurity. To prevent epimerization of the C-3, we designed and synthesized MethylSF2312, ((1,5-dihydroxy-3-methyl-2-oxopyrrolidin-3-yl)phosphonic acid), which contains a fully-substituted C-3 alpha carbon. As a racemic-diastereomeric mixture, MethylSF2312 is equipotent to SF2312 in enzymatic and cellular systems against Enolase. Chiral HPLC separation of a protected MethylSF2312 precursor resulted in the efficient separation of the four enantiomers. After deprotection and inevitable re-equilibration of the anomeric C-5, (3S)-MethylSF2312 was up to 2000-fold more potent than (3R)-MethylSF2312 in an isolated enzymatic assay. This observation strongly correlates with biological activity in both human cancer cells and bacteria for the 3S enantiomer of SF2312. Novel X-ray structures of human ENO2 with chiral and racemic MethylSF2312 show that only (3S,5S)-enantiomer occupies the active site. Enolase inhibition is thus a direct result of binding by the (3S,5S)-enantiomer of MethylSF2312. Concurrent with these results for MethylSF2312, we contend that the (3S,5S)-SF2312 is the single active enantiomer of inhibitor SF2312.

ENOblock Does Not Inhibit the Activity of the Glycolytic Enzyme Enolase

Inhibition of glycolysis is of great potential for the treatment of cancer. However, inhibitors of glycolytic enzymes with favorable pharmacological profiles have not been forthcoming. Due to the nature of their active sites, most high-affinity transition-state analogue inhibitors of glycolysis enzymes are highly polar with poor cell permeability. A recent publication reported a novel, non-active site inhibitor of the glycolytic enzyme Enolase, termed ENOblock (N-[2-[2-2-aminoethoxy)ethoxy]ethyl]4-4-cyclohexylmethyl)amino]6-4-fluorophenyl)methyl]amino]1,3,5-triazin-2-yl]amino]benzeneacetamide). This would present a major advance, as this is heterocyclic and fully cell permeable molecule. Here, we present evidence that ENOblock does not inhibit Enolase enzymatic activity in vitro as measured by three different assays, including a novel 31P NMR based method which avoids complications associated with optical interferences in the UV range. Indeed, we note that due to strong UV absorbance, ENOblock interferes with the direct spectrophotometric detection of the product of Enolase, phosphoenolpyruvate. Unlike established Enolase inhibitors, ENOblock does not show selective toxicity to ENO1-deleted glioma cells in culture. While our data do not dispute the biological effects previously attributed to ENOblock, they indicate that such effects must be caused by mechanisms other than direct inhibition of Enolase enzymatic activity.

Phosphonate Biochemistry

Organophosphonic acids are unique as natural products in terms of stability and mimicry. The C-P bond that defines these compounds resists hydrolytic cleavage, while the phosphonyl group is a versatile mimic of transition-states, intermediates, and primary metabolites. This versatility may explain why a variety of organisms have extensively explored the use organophosphonic acids as bioactive secondary metabolites. Several of these compounds, such as fosfomycin and bialaphos, figure prominently in human health and agriculture. The enzyme reactions that create these molecules are an interesting mix of chemistry that has been adopted from primary metabolism as well as those with no chemical precedent. Additionally, the phosphonate moiety represents a source of inorganic phosphate to microorganisms that live in environments that lack this nutrient; thus, unusual enzyme reactions have also evolved to cleave the C-P bond. This review is a comprehensive summary of the occurrence and function of organophosphonic acids natural products along with the mechanisms of the enzymes that synthesize and catabolize these molecules.

SF2312 is a natural phosphonate inhibitor of enolase

Despite being critical for energy generation in most forms of life, few if any microbial antibiotics specifically inhibit glycolysis. To develop a specific inhibitor of the glycolytic enzyme Enolase 2 for the treatment of cancers with deletion of Enolase 1, we modeled the synthetic tool compound inhibitor, Phosphonoacetohydroxamate (PhAH) into the active site of human ENO2. A ring-stabilized analogue of PhAH, with the hydroxamic nitrogen linked to the alpha-carbon by an ethylene bridge, was predicted to increase binding affinity by stabilizing the inhibitor in a bound conformation. Unexpectedly, a structure based search revealed that our hypothesized back-bone-stabilized PhAH bears strong similarity to SF2312, a phosphonate antibiotic of unknown mode of action produced by the actinomycete Micromonospora, which is active under anaerobic conditions. Here, we present multiple lines of evidence, including a novel X-ray structure, that SF2312 is a highly potent, low nM inhibitor of Enolase.