Synthesis of a new fluorogenic substrate for the assay of phosphoinositide-specific phospholipase C

Fluorogenic approach to evaluating prodrug hydrolysis and stability in live cells

Fluorescein diester which is conjugated with cell membrane permeable Arg9 peptide was proposed as probe for ester prodrug stability and drug release study in living cells. α-Amino protected d-Val and l-Ala which bear differently hindered side chains were used to afford model diesters of 5-maleimide-fluorescein. Such fluorescein diesters were further conjugated with a Cys containing cell membrane permeable Arg9 peptide via thiol-ene crosslink reaction. The resulted conjugates of fluorescein diester and Arg9 peptide were purified with HPLC and characterized with MALDI-TOF MS. Upon incubation with cultured cells, the fluorescein diesters were delivered into the cells, the following hydrolysis of fluorescein diesters and release of fluorescein inside living cells were observed by monitoring the fluorescence accumulation. Fluorescence microscopic imaging studies of HeLa cells treated with fluorescein l-Ala diester show strong fluorescence accumulation in 30 min indicating fast hydrolysis of fluorescein diester and fluorescein release; in contrast d-Val diester remains stable inside cells evidenced by margin fluorescence formation. Further flowcytometry studies on the fluorescein diester-Arg9 conjugate treated cells show that the hydrolysis t_1/2 for l-Ala diester is 15 min. The results also show that Arg9 peptide not only transports the ester probes into cell efficiently but also can retain and concentrate hydrolytic product fluorescein inside cells so that the accumulated fluorescence can be accurately quantified. This fluorogenic probe approach provides feasible applications in dynamic studies on ester prodrug hydrolysis and release, facilitating screening and optimization of prodrug structures in living cell settings.

Phosphorothiolate analogues of phosphatidylinositols as assay substrates for phospholipase C

Accurate measurement of phosphatidylinositol-specific phospholipase C (PI-PLC) activity is important in view of the key role of this enzyme in signal-transduction pathways. In this work we synthesized enantiomerically pure phosphorothiolate analogues of all natural PI-PLC substrates, including those of phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2), 4-phosphate (PI-4-P), 5-phosphate (PI-5-P) and unphosphorylated PI, in both long- and short-chain versions. The enzymatic cleavage of these substrates produces thiol analogues of diacyl glycerol, which can be quantified by UV absorbance after treatment with dipyridyl disulfide. The monodisperse dihexanoyl derivatives are suitable substrates for PI-PLC assay: they give rise to high enzyme activity, and provide excellent linear kinetic responses. For all substrates, we found a good linear correlation between the reaction rate and the amount of enzyme; this indicated the suitability of this assay for enzyme quantification. The short-chain substrates enable the enzyme specificity with variously phosphorylated inositol head groups to be established--unobstructed by substrate aggregation, "scooting" kinetics on micelles, or surface dilution effects. The kinetic results indicated allosteric behavior of PLC for all substrates tested. We found that substrates phosphorylated at the inositol 4-position (phosphorothiolate analogues of PI-4,5-P2 and PI-4-P) displayed very similar kinetic properties, and were cleaved with approximately 20- to 30-fold higher activity than the 4-nonphosphorylated substrates (analogues of PI-5-P and PI). Hence it appears that interactions between the enzyme and the 4-phosphate group of the substrate, but not its 5-phosphate group, is important for PI-PLC catalysis. In addition, the binding affinities of all four substrate types were found to be quite similar; this indicates that the energy of enzyme interaction with the 4-phosphate group is directed almost entirely to catalysis.

Facile synthesis and investigation of 1,8-dioxooctahydroxanthene derivatives as corrosion inhibitors for mild steel in hydrochloric acid solution

For the first time, the corrosion inhibitive performance of some 1,8-dioxooctahydroxanthene derivatives is proposed.

Eggshell/Fe3O4 nanocomposite: novel magnetic nanoparticles coated on porous ceramic eggshell waste as an efficient catalyst in the synthesis of 1,8-dioxo-octahydroxanthene

An eggshell/Fe₃O₄ nanocomposite was synthesized via an economic and novel method, using recycled eggshell biowaste as a starting material and an aqueous solution of FeSO₄ as a coating agent without any additional alkali or a protective atmosphere.

Small molecule inhibitors of phospholipase C from a novel high-throughput screen

Phospholipase C (PLC) isozymes are important signaling molecules, but few small molecule modulators are available to pharmacologically regulate their function. With the goal of developing a general approach for identification of novel PLC inhibitors, we developed a high-throughput assay based on the fluorogenic substrate reporter WH-15. The assay is highly sensitive and reproducible: screening a chemical library of 6280 compounds identified three novel PLC inhibitors that exhibited potent activities in two separate assay formats with purified PLC isozymes in vitro. Two of the three inhibitors also inhibited G protein-coupled receptor-stimulated PLC activity in intact cell systems. These results demonstrate the power of the high-throughput assay for screening large collections of small molecules to identify novel PLC modulators. Potent and selective modulators of PLCs will ultimately be useful for dissecting the roles of PLCs in cellular processes, as well as provide lead compounds for the development of drugs to treat diseases arising from aberrant phospholipase activity.

Efficient one-pot syntheses of dibenzo[a,i]xanthene-diones and evaluation of their antioxidant activity

Facile and convenient procedures for the synthesis of dibenzo[a,i]xanthene-diones have been described. The process employs one-pot condensation of aldehydes, 2-hydroxynaphthalene-1,4-dione, and 2-naphthol/2,7-dihydroxynaphthalene/2,6-dihydroxynaphthalene using catalytic sulfuric acid in water under reflux. Task-specific ionic liquid, 1-butyl-3-methylimidazolium hydrogen sulfate ([bmim]HSO4), has also been found to be an effective catalyst for this transformation. The protocols defined herein prove to be efficient in terms of high yields, operational simplicity, easy workup, and short reaction time. Also, these compounds were screened for antioxidant activity by 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay.

A fluorogenic, small molecule reporter for mammalian phospholipase C isozymes

Phospholipase C isozymes (PLCs) catalyze the conversion of the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)) into two second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This family of enzymes are key signaling proteins that regulate the physiological responses of many extracellular stimuli such as hormones, neurotransmitters, and growth factors. Aberrant regulation of PLCs has been implicated in various diseases including cancer and Alzheimer's disease. How, when, and where PLCs are activated under different cellular contexts are still largely unknown. We have developed a fluorogenic PLC reporter, WH-15, that can be cleaved in a cascade reaction to generate fluorescent 6-aminoquinoline. When applied in enzymatic assays with either pure PLCs or cell lysates, this reporter displays more than a 20-fold fluorescence enhancement in response to PLC activity. Under assay conditions, WH-15 has comparable K(m) and V(max) with the endogenous PIP(2). This novel reporter will likely find broad applications that vary from imaging PLC activity in live cells to high-throughput screening of PLC inhibitors.

General strategy for the preparation of membrane permeable fluorogenic peptide ester conjugates for in vivo studies of ester prodrug stability

To study ester prodrug stability properties in living cells we have conjugated fluorogenic esters to the cell membrane permeable peptide Arg9. The desired conjugates are prepared by coupling N-maleoyl amino acid esters of monoalkylated fluoresceins or fluorescein to TyrArg9Cys. The photophysical properties of the monoalkylated fluorescein derivatives are described.

Synthesis of fluorogenic substrates for continuous assay of phosphatidylinositol-specific phospholipase C

An improved synthesis of fluorogenic substrate analogues for phosphatidylinositol-specific phospholipase C (PI-PLC) is described. The water-soluble substrates, which are derived from fluorescein, are not fluorescent until cleaved by the enzyme, and provide a convenient means to continuously monitor PI-PLC activity. The improvement in the synthesis lies in the method used to protect the hydroxyl groups of the inositol portion of the substrate molecule and allows a milder deprotection procedure to be used. The result is a much more reproducible synthesis of the substrate. The improved procedure has been employed to synthesize a series of fluorogenic substrates, which differ in the length of the aliphatic tail attached to the fluorescein portion of the molecule. The length of the tail was found to have a significant effect on the rate of cleavage of these substrates.

Synthesis of a new fluorogenic substrate for the continuous assay of mammalian phosphoinositide-specific phospholipase C

The synthesis of a fluorogenic substrate for mammalian phosphoinositide-specific phospholipase C is described. The substrate, based on the widely used fluorescein molecule, is a water-soluble substrate analog of phosphatidylinositol-4-phosphate. The fluorogenic substrate 2 is shown to be a sensitive substrate for human PI-PLC-delta1 in a continuous assay.