Bioavailability and bioconcentration potential of perfluoroalkyl-phosphinic and -phosphonic acids in zebrafish (Danio rerio): Comparison to perfluorocarboxylates and perfluorosulfonates

Currently, information regarding bioavailability and bioconcentration potential of perfluoroalkyl phosphinic acids (PFPiAs) in aquatic organisms does not exist. The main objective of the present study was to assess uptake and elimination kinetics of PFPiAs in zebrafish (Danio rerio) following aqueous exposure. The results showed that PFPiA exposure can result in very high steady-state bioconentration factors (BCFss), compared to perfluorocarboxylates and perfluorosulfonates.C6/C10 PFPiA exhibited the highest BCFss, ranging between 10(7) and 10(10), orders of magnitude higher than those for long-chain perfluorocarboxylates. Strong positive relationships were observed between BCFss versus the membrane-water distribution coefficient (Dmw) and the protein-water partition coefficient (Kpw) of the studied perfluoroalkyl substances. However, BCFss exhibited a substantial drop for the very hydrophobic PFPiAs (C8/C10 and C6/C12 PFPiAs). The reduced BCFss of these long-chain PFPiAs (perfluoroalkyl chain length=18; Dmw=10(9)) is likely the result of reduced bioavailability due to interaction with solute molecules/organic matter present in the water phase and/or reduced gill membrane permeability. While PFPiAs can be metabolized to perfluoroalkyl phosphonic acids, the metabolic transformation rate seems insufficient to counteract the high degree of uptake across gill membranes. These findings help to better understand exposure pathways and bioaccumulation behavior of these important perfluorinated acids in aquatic systems.

Production of xylitol and tetrahydrofurfuryl alcohol from xylan in napier grass by a hydrothermal process with phosphorus oxoacids followed by aqueous phase hydrogenation

The production of xylitol and tetrahydrofurfuryl alcohol (THFA) from napier grass was studied using two steps: a hydrothermal process with phosphorus oxoacids followed by aqueous phase hydrogenation with Pd/C. Xylose obtained from the napier grass by the hydrothermal treatment with 3.0 wt% phosphorous acid was subsequently converted into xylitol at 51.6% yield of the xylan in napier grass by hydrogenation with 5.0 wt% Pd/C. The furfural produced from napier grass with a 3.0 wt% phosphoric acid treatment was also directly subjected to the hydrogenation as a hydrolysate to yield 41.4% THFA based on the xylan in napier grass. The yields of xylitol and THFA obtained by hydrogenation using the napier grass hydrolysate containing xylose or furfural were almost the same as those of hydrogenation using commercial materials. To our knowledge, this is the first report on the production of THFA in high yield by hydrogenation directly from biomass hydrolysate.

Carboxylic acid (bio)isosteres in drug design

The carboxylic acid functional group can be an important constituent of a pharmacophore, however, the presence of this moiety can also be responsible for significant drawbacks, including metabolic instability, toxicity, as well as limited passive diffusion across biological membranes. To avoid some of these shortcomings while retaining the desired attributes of the carboxylic acid moiety, medicinal chemists often investigate the use of carboxylic acid (bio)isosteres. The same type of strategy can also be effective for a variety other purposes, for example, to increase the selectivity of a biologically active compound or to create new intellectual property. Several carboxylic acid isosteres have been reported, however, the outcome of any isosteric replacement cannot be readily predicted as this strategy is generally found to be dependent upon the particular context (i.e., the characteristic properties of the drug and the drug-target). As a result, screening of a panel of isosteres is typically required. In this context, the discovery and development of novel carboxylic acid surrogates that could complement the existing palette of isosteres remains an important area of research. The goal of this Minireview is to provide an overview of the most commonly employed carboxylic acid (bio)isosteres and to present representative examples demonstrating the use and utility of each isostere in drug design.

Catabolism and detoxification of 1-aminoalkylphosphonic acids: N-acetylation by the phnO gene product

In Escherichia coli uptake and catabolism of organophosphonates are governed by the phnCDEFGHIJKLMNOP operon. The phnO cistron is shown to encode aminoalkylphosphonate N-acetyltransferase, which utilizes acetylcoenzyme A as acetyl donor and aminomethylphosphonate, (S)- and (R)-1-aminoethylphosphonate, 2-aminoethyl- and 3-aminopropylphosphonate as acetyl acceptors. Aminomethylphosphonate, (S)-1-aminoethylphosphonate, 2-aminoethyl- and 3-aminopropylphosphonate are used as phosphate source by E. coli phn⁺ strains. 2-Aminoethyl- or 3-aminopropylphosphonate but not aminomethylphosphonate or (S)-1-aminoethylphosphonate is used as phosphate source by phnO strains. Neither phn⁺ nor phnO strains can use (R)-1-aminoethylphosphonate as phosphate source. Utilization of aminomethylphosphonate or (S)-1-aminoethylphosphonate requires the expression of phnO. In the absence of phnO-expression (S)-1-aminoethylphosphonate is bacteriocidal and rescue of phnO strains requires the simultaneous addition of d-alanine and phosphate. An intermediate of the carbon-phosphorus lyase pathway, 5′-phospho-α-d-ribosyl 1′-(2-N-acetamidoethylphosphonate), a substrate for carbon-phosphorus lyase, was found to accumulate in cultures of a phnP mutant strain. The data show that the physiological role of N-acetylation by phnO-specified aminoalkylphosphonate N-acetyltransferase is to detoxify (S)-1-aminoethylphosphonate, an analog of d-alanine, and to prepare (S)-1-aminoethylphosphonate and aminomethylphosphonate for utilization of the phosphorus-containing moiety.

Evidence for radical-mediated catalysis by HppE: a study using cyclopropyl and methylenecyclopropyl substrate analogues

(S)-2-Hydroxypropylphosphonic acid epoxidase (HppE) is an unusual mononuclear iron enzyme that catalyzes the oxidative epoxidation of (S)-2-hydroxypropylphosphonic acid ((S)-HPP) in the biosynthesis of the antibiotic fosfomycin. HppE also recognizes (R)-2-hydroxypropylphosphonic acid ((R)-HPP) as a substrate and converts it to 2-oxo-propylphosphonic acid. To probe the mechanisms of these HppE-catalyzed oxidations, cyclopropyl- and methylenecyclopropyl-containing compounds were synthesized and studied as radical clock substrate analogues. Enzymatic assays indicated that the (S)- and (R)-isomers of the cyclopropyl-containing analogues were efficiently converted to epoxide and ketone products by HppE, respectively. In contrast, the ultrafast methylenecyclopropyl-containing probe inactivated HppE, consistent with a rapid radical-triggered ring-opening process that leads to enzyme inactivation. Taken together, these findings provide, for the first time, experimental evidence for the involvement of a C2-centered radical intermediate with a lifetime on the order of nanoseconds in the HppE-catalyzed oxidation of (R)-HPP.

Palatability of selected alpine plant litters for the decomposer Lumbricus rubellus (Lumbricidae)

On alpine pastureland the decline in large-bodied earthworm numbers and biomass after abandonment of management might be the result of a shift from highly palatable grass litter to poorly digestible leaf litter of dwarf shrubs. To test this hypothesis, we analysed nitrogen, phosphorous and total phenolic contents of fresh and aged litter of eight commonly occuring alpine plant species and compared consumption rates of these food sources in a controlled feeding experiment with Lumbricus rubellus (Lumbricidae). Furthermore, we analysed the microbial community structure of aged litter materials to check for a relationship between the microbial characteristics of the different plant litter types and the food choice of earthworms. Plant litters differed significantly in their chemical composition, earthworms, however, showed no preference for any litter species, but generally rejected fresh litter material. Microbial community structures of the litter types were significantly different, but we could find no evidence for selective feeding of L. rubellus. We conclude that L. rubellus is a widespread, adaptable ubiquist, which is able to feed on a variety of food sources differing in quality and palatability, as long as they have been exposed to wheathering.

The effect of Asp54 phosphorylation on the energetics and dynamics in the response regulator protein Spo0F studied by molecular dynamics

The response regulator protein Spo0F acts as an intermediate phospho-messenger in the signal transduction pathway that controls initiation of the differentiation process of sporculation in the bacterium Bacillus subtilis. The regulatory domain of Spo0F contains a triad of three conserved aspartate residues, whereof one aspartate (Asp54) is phosphorylated. Using molecular dynamics simulations, we have studied the changes in flexibility induced by phosphorylation and estimated the free energy cost of introducing a phosphate group at this position using alchemical free energy calculations. The deduced conformational flexibility compares well with experimental NMR results. We find that the apo-conformation of the protein explores a rough energy landscape resulting in a broad population of conformational substates. Phosphorylation of Spo0F reduces protein flexibility, and in particular, the so-called anchor and recognition regions exhibit lower mobility relative to the apo-conformation. Phosphorylation of Asp54 (P-Asp54), in which the apo-structure coordinates to the magnesium ion, results in extension of the sidechain, and depending on which carboxylate oxygen is phosphorylated, distinct interactions between P-Asp54 and magnesium ion as well as residues in its proximity are established. However, phosphorylation does not affect the coordination number of the magnesium ion yielding, within the statistical uncertainties, the same free energy change.

Anaerobic utilization of phosphite and hypophosphite by Bacillus sp

A Bacillus sp. capable of utilizing phosphite and hypophosphite under anaerobic conditions was isolated from Cape Canerval soil samples. The organism was isolated on a glucose-mineral salts medium with phosphate deleted. Anaerobic cultivation of this isolate resulted in decreases in the hypophosphite or phosphite concentration, increases in turbidity, cell count, and dry-cell weight, and decreases in pH and glucose concentration. The optimum hypophosphite concentration for this isolate was 60 microgram/ml, whereas the optimum phosphate concentration was greater than 1,000 microgram/ml, suggesting that higher concentrations of hypophosphite may be toxic to this isolate. Hypophosphite or phosphite utilization was accompanied by little or no detectable accumulation of phosphate in the medium, and 32P-labeled hypophosphite was incorporated into the cell as organic phosphate. When phosphate was present in the medium, the isolate failed to metabolize phosphite. In the presence of phosphite and hypophosphite, the isolate first utilized phosphite and then hypophosphite.

[Effect of cocaine on metabolism of phospholipid in rat vas deferens (author's transl)]

The incorporation of 32P-orthophosphate into phosphatidylethanolamine and phosphatidylcholine in vas deferens was increased by incubation with cocaine and denervation enhanced the action of cocaine on phospholipid metabolism in vas deferens. These results suggested that cocaine directly affects the cell membrane of vas deferens and stimulates phospholipid metabolism.

Alterations of alkaline phosphatase activity during adaptation of Escherichia coli to phosphite and hypophosphite

When Escherichia coli cells were grown in media containing either phosphite or hypophosphite as the sole source of phosphorus, the responded to this situation primarily in the same way as phosphate-limited cultures: The activity of alkaline phosphatase increased drastically, which under natural conditions would enable the cells to compensate for the shortage of phosphate. Subsequent transfers, however, resulted in a quite different response: While the phosphatase activity of phosphate-limited cells stays at a high derepressed level, its increase was followed by a gradual decline in organisms grown on phosphite of hypophosphite. After eight to ten transfers on these P-compounds, phosphatase activity was back to its initial, repressed, low level, indicating that the cells were fully adapted to these substrates. Adaptation to either PO3-3 or PO3-2 was completely abolished if the cells were again grown with PO3-3 as P-source, whereafter the entire process of adaptation had to be repeated. The observed adaptation pattern, reflected by the alterations of phosphatase activity, was qualitatively equal with PO3-3 and PO3-2, but quantitatively different, because the response to hypophosphite gave much higher values than the increase obtained with phosphite. Phosphite-adapted cells are not simultaneously adapted to hypophosphite, but their response to the latter was less intense than observed after direct transfers from PO3-4 to PO3-2. Adaptation to hypophosphite, however, led simultaneously to phosphite adaptation, so that these cells can utilize both P-compounds as a substitute for phosphate.