Synthesis and biological activity of 17 alpha-alkynylamide derivatives of estradiol

Seven estradiol (E2) derivatives with an alkynylamide side chain at the 17 alpha position were synthesized starting from ethynylestradiol (EE2). The main chemical step was the coupling reaction of the acetylide ion of EE2 with carbon dioxide, glutaric anhydride or bromoalkyl ortho ester. The synthesis of these compounds is fast (3-6 steps according to the compound) and is easily achieved with good yield. Five compounds with different side chain lengths were evaluated for uterotrophic and antiuterotrophic activity in the CD-1 mouse. None of the tested compounds shows estrogenic activity in this sensitive in vivo system. At low doses (1 and 3 micrograms), a 14-57% inhibition of E2-induced uterine growth was observed while no additional inhibition was observed at the 10, 20 and 30 micrograms doses. In human breast carcinoma cells in culture, all compounds show estrogenic activity at high concentrations while only compound 39 (N-butyl,N-methyl-8-[3',17' beta-dihydroxy estra-1',3',5'(10')-trien-17' alpha-yl]-7-octynamide) possesses antiproliferative or antiestrogenic effects. No significant correlation could be demonstrated between alkynylamide side chain length and estrogenic or antiestrogenic activity. Among the compounds tested, the derivative of EE2 possessing a five-methylene (CH2) side chain (compound 39) possesses the best antiestrogenic activity (44 +/- 7% in the CD-1 mouse uterus assay at the 3 micrograms dose and 57 +/- 4% at 0.1 nM in human ZR-75-1 cancer cells in culture.     

Humate-induced activation of human granulocytes

Naturally occurring humic substances are particular chemical compounds which are found in humus. They bind to carbohydrates, amino acids and steroids by means of hydrogen bonds, covalent bonds and epsilon donor-acceptor complexes. Three specimens of low-molecular humic substances were tested (two naturally occurring humates and one synthetically prepared humate). They were all capable of stimulating certain functions of human neutrophils (PMN), such as the respiratory burst which results in the production of toxic oxygen compounds. This PMN stimulation can be demonstrated with the help of chemiluminescence, as well as by cytochemistry and with the electron microscope. The main product of the humate-induced PMN response is H2O2. There was no activation of neutrophilic chemokinesis or chemotaxis. It is suggested that the low-molecular humic substances originating from decaying organic material contain chemical structures which can act as signals to change dormant PMN into activated cells.     

Anaerobically grown <Emphasis Type="Italic">Thauera aromatica</Emphasis>, <Emphasis Type="Italic">Desulfococcus multivorans</Emphasis>, <Emphasis Type="Italic">Geobacter sulfurreducens</Emphasis> are more sensitive towards organic solvents than aerobic bacteria

The effect of seven important pollutants and three representative organic solvents on growth of Thauera aromatica K172, as reference strain for nitrate-reducing anaerobic bacteria, was investigated. Toxicity in form of the effective concentrations (EC50) that led to 50% growth inhibition of potential organic pollutants such as BTEX (benzene, toluene, ethylbenzene, and xylene), chlorinated phenols and aliphatic alcohols on cells was tested under various anaerobic conditions. Similar results were obtained for Geobacter sulfurreducens and Desulfococcus multivorans as representative for Fe³⁺-reducing and sulphate-reducing bacteria, respectively, leading to a conclusion that anaerobic bacteria are far more sensitive to organic pollutants than aerobic ones. Like for previous studies for aerobic bacteria, yeast and animal cell cultures, a correlation between toxicity and hydrophobicity (log P values) of organic compounds for different anaerobic bacteria was ascertained. However, compared to aerobic bacteria, all three tested anaerobic bacteria were shown to be about three times more sensitive to the tested substances.     

SOS Chromotest study concerning some appreciation criteria of humic substances' genotoxic potency

The genotoxicity of 5 compounds: 2 fulvic acids, a trade humic acid, a synthetic humic material (SHM), and 2,5-dihydroxybenzoic acid, was assessed after chlorination, by means of the SOS Chromotest for tester strain E. coli PQ 37 without metabolic activation. Chlorination was carried out for humic material concentration of 0.5 mg total organic carbon per liter, and chlorine concentrations in the range of 0.1-2.0 chlorine equivalents per mole of carbon. Among all the considered criteria that can account for potent toxicity: chemical degradation determined by the UV absorption decrease, chlorine consumption, average molecular weight, only the polymerization index (O.D. 665 nm/O.D. 465 nm) can be related to the genotoxicity of humic samples. This latter criterion appears a possible predictor of genotoxic potency, revealed subsequent to the aqueous chlorination of humic materials. Looking at the various genotoxic activities of the tested compounds, SHM can be considered a better model of natural humic materials than the trade humic acid.     

UPTAKE OF CADMIUM BY FRESHWATER GREEN ALGAE: EFFECTS OF PH AND AQUATIC HUMIC SUBSTANCES1

The effects of humic substances and low pH on short‐term Cd uptake by Pseudokirchneriella subcapitata (Korshikov) Hindak and Chlamydomonas reinhardtii Dang were investigated under defined exposure conditions. The uptake experiments were run in the presence of either a synthetic organic ligand (nitrilotriacetate) or natural organic ligands (Suwannee River fulvic or humic acid). An ion‐exchange method was used to measure the free Cd²⁺ concentrations in the exposure solutions. At pH 5, measured free Cd²⁺ concentrations agreed with estimations made using the geochemical equilibrium model WHAM, but at pH 7 the model overestimated complexation by both Suwannee River fulvic and humic acids compared with the ion‐exchange measurements. Consistent with the metal internalization step being rate limiting for overall short‐term uptake, intracellular Cd uptake was linear for exposure times less than 20 min at pH 5 or pH 7 for both algal species. After taking into account complexation of Cd in solution, Suwannee River humic substances had no additional effects on cadmium uptake at pH 7, as would be predicted by the free ion model. This absence of effects other than complexation persisted at pH 5, where the tendency of humic substances to adsorb to the algal cell surface is favored. Changes in pH strongly influenced Cd uptake, with the intracellular flux of Cd being at least 20 times lower at pH 5 than at pH 7 for P. subcapitata. Our results support models such as the free ion model or the biotic ligand model, in which humic substances act indirectly on Cd uptake by reducing the bioavailability of Cd by complexation in solution.     

Inhibition of phosphatase activity by dissolved humic substances and hydrolytic reactivation by natural ultraviolet light

1. Phosphatases released extracellularly by aquatic micro-organisms often complex with humic compounds that are released from decomposing tissues of plants and imported in dissolved and colloidal forms to lakes and rivers. 2. Dissolved humic substances from several natural sources formed complexes with phosphatases of bacterial and algal origin and reduced hydrolytic activity by non-competitive inhibition. Restoration of the hydrolytic enzyme activities from the humic substances–enzyme complexes increased progressively over time when exposed experimentally to natural and artificial ultraviolet (UV) irradiance. 3. Greater phosphatase restoration occurred from humic acid–phosphatase complexes when humic acids were extracted from dissolved organic matter (DOM) of mixed natural plant sources, than when humic acids were isolated from a decomposing single plant species. 4. The data support a previously suggested hypothesis that phosphatases and other enzymes in aquatic ecosystems can complex with humic substances that dominate the DOM pool. These humic substances–enzyme complexes, in which the enzyme is temporarily inactivated, can be transported with water movements and displaced to other sites within the ecosystem. Upon exposure to UV irradiance in the photic zone, functional enzymes can be released. The potential for inactivation and storage of enzyme activity, relocation within the ecosystem, and subsequent reactivation holds important implications for regulation of nutrient cycling in fresh waters.     

Soil humic substances affect transport properties of tonoplast vesicles isolated from oat roots

The effect of a low molecular size (<5 KDa) humic fraction, essentially fulvic acids, on microsomal and tonoplast ion-stimulated ATPase activity was studied. After 20 min of pre-incubation with microsomal vesicles from oat roots, humic substances at organic C concentration of up to 0.5 μg cm^-3 increased KCl-stimulated ATPase activity, while they inhibited enzyme activity at higher concentrations. Cl^--stimulated ATPase activity of tightly sealed tonoplast-enriched vesicles was similarly affected by <5 KDa humic substances. This behaviour was not observed when gramicidin D was added to the assay medium. Proton transport by vesicles incubated up to 5 min with <5 KDa humic molecules was affected in a concentration-dependent manner, strongly resembling that observed for ATP hydrolysis, whereas it was severely reduced when the assay conditions were close to those used for measuring ATP hydrolysis (20 min pre-incubation of vesicles with humic substances). The transmembrane electrical potential was negatively affected, irrespective of the concentration of humic molecules. Furthermore, a 15-min pre-incubation strongly reduced the formation of a potential gradient. The size and concentrations of humic substances employed make an interaction with the vacuolar membrane of root cells plausible. The results show that the main target of humic molecules is the electrical membrane potential and suggest a possible way of interference of these naturally occurring substances with the biochemical mechanisms involved in plant mineral nutrition.     

Pyrolysis-field ionization mass spectrometry of agricultural soils and humic substances: Effect of cropping systems and influence of the mineral matrix

Whole soil samples, extracted humic substances, the corresponding fulvic (FA) and humic acids (HA) and the extraction residues (humins) from long-term, agricultural test plots were investigated by in-source pyrolysis-field ionization mass spectrometry (Py-FIMS). For the soils distinct differences in the chemical composition of the organic matter in differently managed fields were observed. The FI mass spectra of the extracted humic substances gave complementary chemical information, as they cover a larger mass range compared to the whole soil spectra. The chemical, structural information of the conventional alkaline extraction residues was demonstrated by Py-FIMS spectra to be similar to that of the related soil samples. Influences of mineral matrix to organic matter ratios were studied on mixtures of extracted humic substances with defined mineral components such as quartz, basalt, iron oxide (Fe₂O₃), Ca-montmorillonite, kaolinite and illite. It was shown that in these mixtures the number of mass signals detected and the covered mass range decreased, when organic carbon concentrations (C_org) in this synthetic mineral matrix dropped below 2% (w/w). Limitations in the direct application of Py-FIMS might arise in the case of natural soil samples with C_org concentrations below 0.5% (w/w), high contents of swelling clay minerals and iron oxides. ei]{gnR}{fnMerckx}     

Direct superoxide anion scavenging by a disodium disuccinate astaxanthin derivative: Relative efficacy of individual stereoisomers versus the statistical mixture of stereoisomers by electron paramagnetic resonance imaging

Carotenoids are a related group of greater than 600 natural compounds, irrespective of geometric- and stereoisomers, with demonstrated antioxidant efficacy. The carotenoids are broadly divided into "carotenes," or non-oxygen substituted hydrocarbon carotenoids, and "xanthophylls," oxygen-substituted carotenoids. The natural compounds are excellent singlet oxygen quenchers as well as lipid peroxidation chain-breakers; this dual antioxidant capacity is generally attributed to the activity of the polyene chain, and increases with the number of conjugated double bonds along the polyene chain length. However, the poor aqueous solubility of most carotenes and the vast majority of xanthophylls limits their use as aqueous-phase singlet oxygen quenchers and direct radical scavengers. A variety of introduction vehicles (e.g., organic solvents, cyclodextrins) have been used to introduce the insoluble carotenoids into aqueous test systems. Hawaii Biotech, Inc. (HBI) successfully synthesized a novel carotenoid derivative, the disodium disuccinate derivative of astaxanthin (3,3(')-dihydroxy-beta,beta-carotene-4,4(')-dione) in all-trans (all-E) form. The novel derivative is a water-dispersible symmetric chiral molecule with two chiral centers, yielding four stereoisomeric forms: 3R,3(')R and 3S,3(')S (enantiomers), and the diastereomeric meso forms (3R,3(')S and 3(')R,3S). The individual stereoisomers were synthesized at high purity (>90% by HPLC) and compared directly for efficacy with the statistical mixture of stereoisomers obtained from the synthesis from the commercial source of astaxanthin (1:2:1 ratio of 3S,3(')S, meso, and 3R,3(')R, respectively). Direct scavenging of superoxide anion was evaluated in a standard in vitro isolated human neutrophil assay by electron paramagnetic resonance (EPR) imaging, employing the spin-trap DEPMPO. Each novel derivative was tested in pure aqueous formulation and in ethanolic formulation shown to completely disaggregate the compounds in solution. In each case, the ethanolic formulation was a more potent scavenging vehicle. No significant differences in scavenging efficiency were noted among the individual stereoisomers and the statistical mixture of stereoisomers, suggesting that the polyene chain alone was responsible for superoxide scavenging. Dose-ranging revealed that the statistical mixture of stereoisomers of the novel derivative, at millimolar (mM) concentrations, could nearly completely eliminate the superoxide anion signal generated in the activated human neutrophil assay. All ethanolic formulations of the novel derivatives exhibited increased scavenging efficiency over equimolar concentrations of non-esterified astaxanthin delivered in a dimethyl sulfoxide (DMSO) vehicle. These novel compounds will likely find utility in applications requiring aqueous delivery of a highly potent direct radical scavenger.     

Responses of the toxic cyanobacterium Microcystis aeruginosa to iron and humic substances.

Iron is an essential element to marine biota. Different types of dissolved organic matter (DOM), such as humic substances have impacts on the marine coastal waters iron chemistry. The aim of the study was to examine how the presence of humic substances (both aquatic and sedimentary) may affect iron bioavailability to the bloom-forming cyanobacterium Microcystis aeruginosa Kutzing incubated on standard and modified mineral BG-11 media. The final iron concentrations in the growth media ranged from 0.1 to 100microM. The results demonstrate that both the growth rate and the concentration of chlorophyll a in cultures of M. aeruginosa are limited by insufficient (<10microM) Fe concentrations. The addition of aquatic humic substances in the presence of iron in concentrations <0.1microM increased the optical density 25-fold, and the production of chlorophyll a 15-fold as compared with the cultures exposed to iron only at the same concentration. Sedimentary humic acids in the presence of iron at a concentration of 10microM reduced the growth and production of chlorophyll a by 50% as compared to the cultures exposed to iron only at the same concentration. Possible mechanisms of humic substances - metal ion - alga interactions are discussed. It is suggested that aquatic humic substances could be of great importance in the formation of cyanobacteria blooms.     

Effects of dissolved organic matter (DOM) at environmentally relevant carbon concentrations on atrazine degradation by Chelatobacter heintzii SalB

The dissolved organic matter (DOM) is the term used for organic components of natural origin present in the soil solution and is probably the most available C-source that primes microbial activity in subsoils. Contrasting effects of organic C components on pesticide degradation have been reported; however, most studies have used model organic compounds with compositions and concentrations which differ substantially from those found in the environment. Degradation of atrazine (AT) by Chelatobacter heintzii SalB was monitored in liquid batch assays in the absence or presence of well-defined model C compounds (glucose, gluconate and citrate) as model DOM (mDOM) or complex, less-defined, environmental DOM solutions (eDOM: isolated humic substances, soil and plant residue extracts) at environmentally relevant concentrations. Glucose significantly increased AT degradation rate by more than a factor of 8 at and above 2.5?mg C L(?-?1). Optical density measurements showed that this stimulation is related to microbial growth. Gluconate and citrate had no effects unless at non-relevant concentrations (1,000?mg DOC L(?-?1)) at which stimulations (gluconate) or inhibitions (citrate) were found. The effects of eDOM added at 10?mg DOC L(?-?1) on AT degradation were generally small. The AT degradation time was reduced by factors 1.4-1.9 in the presence of humic acids and eDOM from soils amended with plant residues; however, no effects were found for fulvic acids or eDOM from a soil leachate solution or extracted from unamended peat or forest soil. In conclusion, DOM supplied as both mDOM and eDOM did not inhibit AT degradation at environmentally relevant concentrations, and stimulation can be found for selected DOM samples and this is partly related to its effect on growth.     

Estrogens can disrupt amphibian mating behavior

The main component of classical contraceptives, 17α-ethinylestradiol (EE2), has high estrogenic activity even at environmentally relevant concentrations. Although estrogenic endocrine disrupting compounds are assumed to contribute to the worldwide decline of amphibian populations by adverse effects on sexual differentiation, evidence for EE2 affecting amphibian mating behaviour is lacking. In this study, we demonstrate that EE2 exposure at five different concentrations (0.296 ng/L, 2.96 ng/L, 29.64 ng/L, 2.96 μg/L and 296.4 μg/L) can disrupt the mating behavior of adult male Xenopus laevis. EE2 exposure at all concentrations lowered male sexual arousal, indicated by decreased proportions of advertisement calls and increased proportions of the call type rasping, which characterizes a sexually unaroused state of a male. Additionally, EE2 at all tested concentrations affected temporal and spectral parameters of the advertisement calls, respectively. The classical and highly sensitive biomarker vitellogenin, on the other hand, was only induced at concentrations equal or higher than 2.96 μg/L. If kept under control conditions after a 96 h EE2 exposure (2.96 μg/L), alterations of male advertisement calls vanish gradually within 6 weeks and result in a lower sexual attractiveness of EE2 exposed males toward females as demonstrated by female choice experiments. These findings indicate that exposure to environmentally relevant EE2 concentrations can directly disrupt male mate calling behavior of X. laevis and can indirectly affect the mating behavior of females. The results suggest the possibility that EE2 exposure could reduce the reproductive success of EE2 exposed animals and these effects might contribute to the global problem of amphibian decline.     

腐殖质在环境污染物生物降解中的作用研究进展

腐殖质物质在地球的生态环境中大量存在,它不仅可以在有毒化合物的生物降解和生物转化过程中起到氧化还原中间体的作用,加速有毒物质的降解和转化。也可以作为唯一末端电子受体,接受来自一些有机酸或者甲苯等环境中有毒物质提供的电子,偶联能量的产生,支持菌体的生长,形成一种新的细菌厌氧呼吸形式——腐殖质呼吸。因此,对腐殖质在环境有毒物质的生物降解和生物转化过程中的作用进行研究,不仪对于深入理解细菌呼吸的本质具有重要的理论意义,而且对于环境有毒物质的降解和转化以及元素的生物地球化学循环具有重要的生态学意义,同时对地球表面的有毒物质进行更有效的生物降解具有重要的现实意义。     

Effects of humic materials on virus recovery from water

Humic and fulvic acids were tested for their ability to interfere with virus recovery by microporous filters. Two electropositively charged types of filter (Seitz S and Zeta Plus 60S) were used to concentrate poliovirus in the presence of humic materials. Humic acid inhibited virus adsorption, but even at the highest humic acid concentrations tested (200 mg/liter), 30 to 40% of the virus was recovered by the filters. Fulvic acid, tested with Zeta Plus filters, did not affect virus recovery. For comparison, two electronegatively charged filter types were tested (Cox and Balston). These two types of filter were more sensitive to interference at lower concentrations of humic acid than the more positively charged filters. With Balston filters, at humic acid concentrations above 10 mg/liter, most of the virus was recovered in the filtrate. Fulvic acid, tested with Balston filters, did not interfere with virus recovery. With the electropositively charged filters, the humic materials adsorbed efficiently, even at high input concentrations. Interference with virus adsorption occurred at humic acid concentrations which were below the level of saturation of the filters. In addition, in high-volume experiments, humic acid led to premature blockage of the filters. The efficiency of virus recovery by a second concentration step, organic flocculation of the filter eluate, was tested. For all the filter types tested, this procedure was not affected by the presence of humic or fulvic acid in the input water.     

Effects of humic materials on virus recovery from water.

Humic and fulvic acids were tested for their ability to interfere with virus recovery by microporous filters. Two electropositively charged types of filter (Seitz S and Zeta Plus 60S) were used to concentrate poliovirus in the presence of humic materials. Humic acid inhibited virus adsorption, but even at the highest humic acid concentrations tested (200 mg/liter), 30 to 40% of the virus was recovered by the filters. Fulvic acid, tested with Zeta Plus filters, did not affect virus recovery. For comparison, two electronegatively charged filter types were tested (Cox and Balston). These two types of filter were more sensitive to interference at lower concentrations of humic acid than the more positively charged filters. With Balston filters, at humic acid concentrations above 10 mg/liter, most of the virus was recovered in the filtrate. Fulvic acid, tested with Balston filters, did not interfere with virus recovery. With the electropositively charged filters, the humic materials adsorbed efficiently, even at high input concentrations. Interference with virus adsorption occurred at humic acid concentrations which were below the level of saturation of the filters. In addition, in high-volume experiments, humic acid led to premature blockage of the filters. The efficiency of virus recovery by a second concentration step, organic flocculation of the filter eluate, was tested. For all the filter types tested, this procedure was not affected by the presence of humic or fulvic acid in the input water.     

UV spectroscopy: a tool for monitoring humification and for proposing an index of the maturity of compost

Composting organic matter is an interesting way to valorize waste. Compost is a product obtained after a humification process. The humification of organic matter during composting was studied by the quantification and monitoring of the evolution of humic substances. The final objective was to be able to evaluate the state of humification based on a spectrum of total humic substances using the method of UV spectral deconvolution. This study presented a new index obtained by UV spectroscopy using the deconvolution of an unknown spectrum of compost with 3 reference spectra. This index was compared to the maturity indices usually used, such as total extraction of humic substances (rate of extraction), IP (index of polymerization) or the humic acid to fulvic acid ratio. This new parameter, which was easy and quick to determine, gave precise information about the quality of the compost. It made it possible to disregard the values of aberrant concentrations caused by the classical protocol of extraction. Compared to the humic acid to fulvic acid ratio, the new index obtained by UV spectral deconvolution showed more representative results: the humic acid to fulvic acid ratio for an apparently non-mature compost was abnormally high, suggesting that the compost was mature, whereas the UV index proposed showed that the compost was really young and not yet humified.     

Modification of PAHs Biodegradation with Humic Compounds

The effect of extractable humic substances (EHS) on the bioremediation of phenanthrene in a slurry phase was investigated using adapted microorganisms with polycyclic aromatic hydrocarbons (PAHs). Two concentrations of EHS were used: 150 and 30 mg/kg soil. The phenanthrene concentration was 500 mg/kg soil. The results showed that the trend of biodegradation was increased after four weeks retardation. These tests showed that humic compounds could overcome the bond between the soil and phenanthrene in the presence of the bacterial consortium. The bacterial density in the medium with EHS was about six-fold greater in magnitude than in the medium without the humic compounds. The chemical relationship between phenanthrene and the humic substances in the form of a phenanthrene-humic-soil complex or phenanthrene-humic is loosely associated and reversible. Therefore, after the initial inhibition by humic substances, the bioavailability of phenanthrene increases.     

腐殖质呼吸作用及其生态学意义

腐殖质呼吸是厌氧环境中普遍存在的一种微生物呼吸代谢模式.自1996年发现以来,日益成为生态学与环境科学领域的研究热点.在厌氧条件下,一些微生物能以腐殖质作为唯一电子受体,氧化环境中的有机质或者甲苯等环境有毒物质,产生CO2,参与碳循环;同时,腐殖质呼吸作用产生的还原态腐殖质可以还原环境中的一些氧化态物质,如Fe(III)、Mn(IV)、Cr(VI)、U(VI) 、硝基芳香化合物和多卤代污染物.因此,腐殖质呼吸能够影响环境中C、N、Fe、Mn以及一些痕量金属元素的生物地球化学循环,并且能够促进重金属以及有机污染物的脱毒,在水体自净、污染土壤原位修复、污水处理等方面具有积极作用.     

Chaetominine, (+)-alantrypinone,questin, isorhodoptilometrin,and 4-hydroxybenzaldehyde produced by the endophytic fungus Aspergillus sp. YL-6 inhibit wheat (Triticum aestivum) and radish (Raphanus sativus) germination

Five phytotoxic substances with allelopathic activity were isolated from endophytic fungi Aspergillus sp. YL-6 habited in Pleioblastus amarus. The chemical structures of these substances were determined as chaetominine (1), (+)-alantrypinone (2), questin (3), isorhodoptilometrin (4), and 4-hydroxybenzaldehyde (5) by nuclear magnetic resonance and mass spectrometry data. The potential allelopathic effects of compounds 1–5 were evaluated on wheat (Triticum aestivum) and radish (Raphanus sativus). Under lab condition, at concentrations of 10 and 20 ppm, compounds 1–5 inhibit the germination and growth of the two tested seeds completely. An idole-3-acetic acid (IAA) derivative, (+)-alantrypinone (2) displayed the best inhibitory effects on radish seeds among these tested compounds with the similar activity as the positive control glyphosate, a broad-spectrum systemic herbicide. In the further evaluation of compounds 1–5, Questin (3), an anthraquinone derivative, can inhibit shoot and root elongation of wheat, the inhibitory effects assessed were similar to the positive control glyphosate.     

Influence of dissolved humic acid on hydrophobic chemical uptake in juvenile rainbow trout

This study measured the chemical uptake of three hydrophobic chemicals (1,2,4-trichlorobenzene (1,2,4-TCB), 1,2,3,4,5-pentachlorobenzene (PeCB) and 2,2',4,4',6,6'-hexachlorobiphenyl (HCBP) with differing octanol-water partition coefficients (log K(ow) values of 3.95, 5.05 and 7.55, respectively) in juvenile rainbow trout (Oncorhynchus mykiss) after 2-day and 4-day aqueous exposures. Because of the affinity of hydrophobic compounds for dissolved organic carbon (DOC) and previous work demonstrating that fish gills take up these three hydrophobic chemicals, we predicted that chemical uptake into the fish would be lowered by the addition of humic acid to the water (1.54, 4.81 and 14.3 mg/l) compared with control fish (no humic acid added to the water). As predicted, humic acid concentrations of >or=4.81 mg/l significantly reduced the whole body concentrations of all three chemicals when compared with 1.54 mg/l humic acid. This effect of humic acid was greatest for HCBP, the chemical with the highest log K(ow), such that chemical uptake was reduced by 3.4-fold for 14.3 mg/l humic acid compared with the control exposure. However, an unexpected finding was that, compared with the control exposure, the lowest concentration of humic acid tested (1.54 mg/l humic acid) significantly increased chemical uptake by up to 112% for the two chemicals with the lower log K(ow), PeCB and 1,2,4-TCB, and did not affect uptake of the high log K(ow) chemical HCBP. We conclude that the ability of DOC to inhibit aqueous uptake of hydrophobic chemicals was dependent on both the concentration of DOC and the log K(ow) of the chemical, but that low humic acid concentrations of approximately 1.5 mg/l can significantly increase uptake of certain chemicals with a log K(ow) between 4 and 5.