Freshwater salinization syndrome: from emerging global problem to managing risks

Freshwater salinization is an emerging global problem impacting safe drinking water, ecosystem health and biodiversity, infrastructure corrosion, and food production. Freshwater salinization originates from diverse anthropogenic and geologic sources including road salts, human-accelerated weathering, sewage, urban construction, fertilizer, mine drainage, resource extraction, water softeners, saltwater intrusion, and evaporative concentration of ions due to hydrologic alterations and climate change. The complex interrelationships between salt ions and chemical, biological, and geologic parameters and consequences on the natural, social, and built environment are called Freshwater Salinization Syndrome (FSS). Here, we provide a comprehensive overview of salinization issues (past, present, and future), and we investigate drivers and solutions. We analyze the expanding global magnitude and scope of FSS including its discovery in humid regions, connections to human-accelerated weathering and mobilization of ‘chemical cocktails.’ We also present data illustrating: (1) increasing trends in salt ion concentrations in some of the world’s major freshwaters, including critical drinking water supplies; (2) decreasing trends in nutrient concentrations in rivers due to regulations but increasing trends in salinization, which have been due to lack of adequate management and regulations; (3) regional trends in atmospheric deposition of salt ions and storage of salt ions in soils and groundwater, and (4) applications of specific conductance as a proxy for tracking sources and concentrations of groups of elements in freshwaters. We prioritize FSS research needs related to better understanding: (1) effects of saltwater intrusion on ecosystem processes, (2) potential health risks from groundwater contamination of home wells, (3) potential risks to clean and safe drinking water sources, (4) economic and safety impacts of infrastructure corrosion, (5) alteration of biodiversity and ecosystem functions, and (6) application of high-frequency sensors in state-of-the art monitoring and management. We evaluate management solutions using a watershed approach spanning air, land, and water to explore variations in sources, fate and transport of different salt ions (e.g. monitoring of atmospheric deposition of ions, stormwater management, groundwater remediation, and managing road runoff). We also identify tradeoffs in management approaches such as unanticipated retention and release of chemical cocktails from urban stormwater management best management practices (BMPs) and unintended consequences of alternative deicers on water quality. Overall, we show that FSS has direct and indirect effects on mobilization of diverse chemical cocktails of ions, metals, nutrients, organics, and radionuclides in freshwaters with mounting impacts. Our comprehensive review suggests what could happen if FSS were not managed into the future and evaluates strategies for reducing increasing risks to clean and safe drinking water, human health, costly infrastructure, biodiversity, and critical ecosystem services.

     

Synthesis and DNA binding properties of saturated distamycin analogues

A series of saturated heterocyclic analogues of distamycin were prepared and examined. A fluorescent intercalator displacement (FID) assay conducted on p[dA]-p[dT] DNA to obtain C(50) values and a hairpin deoxyoligonucleotide containing an A/T-rich binding site was used to evaluate DNA binding affinity. It is observed that saturated heterocycles greatly reduce the DNA binding relative to distamycin.     

Conformationally restricted analogues designed for selective inhibition of GAR Tfase versus thymidylate synthase or dihydrofolate reductase

The synthesis and evaluation of a series of conformationally restricted analogues of 10-formyl-tetrahydrofolate as potential inhibitors of glycinamide ribonucleotide transformylase (GAR Tfase) or aminoimidazole carboxamide transformylase (AICAR Tfase) are reported.     

Substructural specificity and polyvalent carbohydrate recognition by the Entamoeba histolytica and rat hepatic N- acetylgalactosamine/galactose lectins

Both the Entamoeba histolytica lectin, a virulence factor for the causative agent of amebiasis, and the mammalian hepatic lectin bind to N-acetylgalactosamine (GalNAc) and galactose (Gal) nonreducing termini on oligosaccharides, with preference for GalNAc. Polyvalent GalNAc- derivatized neoglycoproteins have >1000-fold enhanced binding affinity for both lectins (Adler,P., Wood,S.J., Lee,Y.C., Lee,R.T., Petri,W.A.,Jr. and Schnaar,R.L.,1995, J. Biol. Chem ., 270, 5164-5171). Substructural specificity studies revealed that the 3-OH and 4-OH groups of GalNAc were required for binding to both lectins, whereas only the E.histolytica lectin required the 6-OH group. Whereas GalNAc binds with 4-fold lower affinity to the E.histolytica lectin than to the mammalian hepatic lectin, galactosamine and N-benzoyl galactosamine bind with higher affinity to the E. histolytica lectin. Therefore, a synthetic scheme for converting polyamine carriers to poly-N-acyl galactosamine derivatives (linked through the galactosamine primary amino group) was developed to test whether such ligands would bind the E.histolytica lectin with high specificity and high affinity. Contrary to expectations, polyvalent derivatives including GalN6lys5, GalN4desmosine, GalN4StarburstTMdendrimer, and GalN8StarburstTMdendrimer demonstrated highly enhanced binding to the mammalian hepatic lectin but little or no enhancement of binding to the E.histolytica lectin. We propose that the mammalian hepatic lectin binds with greatest affinity to GalNAc "miniclusters," which mimic branched termini of N-linked oligosaccharides, whereas the E.histolytica lectin binds most effectively to "maxiclusters," which may mimic more widely spaced GalNAc residues on intestinal mucins.      

Interaction of mouse submaxillary gland renin with a statine-containing, subnanomolar, competitive inhibitor

The interaction between mouse submaxillary gland renin and a statine-containing, iodinated substrate analog inhibitor was studied. The compound, 1 (Boc-His-Pro-Phe-(4-iodo)-Phe-Sta-Leu-Phe-NH2, Sta = (3S,4S)-4-amino-3-hydroxy-6-methyl-heptanoic acid), a statine-containing analog of the renin substrate octapeptide, was a competitive inhibitor of cleavage of synthetic tetradecapeptide renin substrate by mouse submaxillary gland renin, with a Ki of 6.2 x 10(-10) M (pH 7.2, 37 degrees C). Titration of the partial quenching of the tryptophan fluorescence of the enzyme by 1 revealed tight binding with a dissociation constant less than 3 nM and a binding stoichiometry of one mole 1 per mole enzyme. The time course of tight binding of 1 to mouse renin appeared to be fast, with kON greater than or equal to 1.3 x 10(6) s-1 M-1. The UV difference spectrum generated upon binding of 1 to mouse renin had two prominent features: a strong, broad band that had a minimum at 242 nm with delta epsilon (242) = -19,500 cm-1 M-1, and a triplet of enhanced bands centered at 286 nm with delta epsilon (286) about +1100 cm-1 M-1. The strong, broad, negative band was similar to the difference between the UV absorbance of 1 in methanol and in 0.1 M citrate phosphate pH 7.2. A structure-activity correlation for analogs of 1 showed some moieties of 1 that are important for potent inhibition of mouse renin. The inhibition data for these compounds versus human kidney renin suggested that the solution of the crystal structure of 1 bound to mouse renin will provide useful information for the design of inhibitors of human kidney renin.     

Resolution of a CPzI precursor, synthesis and biological evaluation of (+) and (-)-N-Boc-CPzI: a further validation of the relationship between chemical solvolytic stability and cytotoxicity

The chemical resolution, using N-tosyl-L-proline as a chiral auxiliary, of a racemate of the pyrazole analog (+/-)-N-Boc-CPzI of the left hand segment (CPI) of the antitumor agent CC-1065, and the cytotoxic evaluation of both enantiomers are described. The reported results further validate the direct relationship between chemical solvolytic stability of the cyclopropane ring and cytotoxicity proposed by Boger and coworkers.     

Synthesis and evaluation of a series of C5′-substituted duocarmycin SA analogs

The synthesis and evaluation of a key series of analogs of duocarmycin SA, bearing a single substituent at the C5′ position of the DNA binding subunit, are described.     

Correlation of inhibitor effects on enzyme activity and thermal stability for the integral membrane protein fatty acid amide hydrolase

The melting curves of fatty acid amide hydrolase (FAAH) in the presence of 29 reversible inhibitors were measured using a thiol-reactive fluorophore. The thermal stability (T(m)) of the FAAH/inhibitor complex varied significantly depending on the chemical characteristics of the inhibitors, notably variations in the head group. Two separate distributions were observed when T(m) was plotted against K(i). The majority of the inhibitors showed a positive correlation between binding affinity and T(m), however inhibitors with a pyridine carboxylic acid moiety in the head group fell in a distinct and uncorrelated distribution when tail groups were varied.     

X-ray studies of aspartic proteinase-statine inhibitor complexes

The conformation of a statine-containing renin inhibitor complexed with the aspartic proteinase from the fungus Endothia parasitica (EC 3.4.23.6) has been determined by X-ray diffraction at 2.2-A resolution (R = 0.17). We describe the structure of the complex at high resolution and compare this with a 3.0-A resolution analysis of a bound inhibitor, L-364,099, containing a cyclohexylalanine analogue of statine. The inhibitors bind in extended conformations in the long active-site cleft, and the hydroxyl of the transition-state analogue, statine, interacts strongly with the catalytic aspartates via hydrogen bonds to the essential carboxyl groups. This work provides a detailed structural analysis of the role of statine in peptide inhibitors. It shows conclusively that statine should be considered a dipeptide analogue (occupying P1 to P1') despite lacking the equivalent of a P1' side chain, although other inhibitor residues (especially P2) may compensate by interacting at the unoccupied S1' specificity subsite.