A roadmap for high-resolution satellite soil moisture applications – confronting product characteristics with user requirements

Soil moisture observations are of broad scientific interest and practical value for a wide range of applications. The scientific community has made significant progress in estimating soil moisture from satellite-based Earth observation data, particularly in operationalizing coarse-resolution (25-50 km) soil moisture products. This review summarizes existing applications of satellite-derived soil moisture products and identifies gaps between the characteristics of currently available soil moisture products and the application requirements from various disciplines. We discuss the efforts devoted to the generation of high-resolution soil moisture products from satellite Synthetic Aperture Radar (SAR) data such as Sentinel-1 C-band backscatter observations and/or through downscaling of existing coarse-resolution microwave soil moisture products. Open issues and future opportunities of satellite-derived soil moisture are discussed, providing guidance for further development of operational soil moisture products and bridging the gap between the soil moisture user and supplier communities

Metabolism of the phenylpropanoids methyleugenol, (E\it E )-methylisoeugenol, alpha-, beta- und gamma-asarone

Methyleugenol (ME), ($\it E$)-Methylisoeugenol (MIE), alpha-Asaron (aA), beta-Asaron (bA) und gamma-Asaron (gA) sind natürlich vorkommende Pflanzeninhaltsstoffe aus der Klasse der Phenylpropene (PP) und Bestandteil der menschlichen Ernährung. ME, aA und bA erwiesen sich im Tierversuch als hepatokanzerogen. MIE und gA wurden bislang nicht untersucht. Die allylischen Hepatokanzerogene Estragol und Safrol werden im Verlauf des Fremdstoff-metabolismus durch Cytochrom P450-Enzyme (CYP) in 1‘-Position hydroxyliert und anschließend durch Sulfotransferasen sulfoniert. Der reaktive Schwefelsäureester kann DNA-Addukte bilden, wodurch in der Folge Mutationen und Tumoren ausgelöst werden können. Für ME wurde der gleiche Mechanismus der Aktivierung postuliert, obgleich definitive Beweise bislang fehlten. Im Gegensatz dazu war die Mehrheit der bislang untersuchten propenylischen PP nicht kanzerogen. aA und bA stellen insofern eine Ausnahme dar. Tierexperimentelle Untersuchungen legen einen alternativen Mechanismus für die Aktivierung von aA und bA nahe. Untersuchungen zum Metabolismus der Asarone liegen bisher nicht vor. Ziel der Arbeit war es daher, den hepatischen Metabolismus dieser fünf PP zu untersuchen, um Einblicke in die potentiellen gentoxischen Mechanismen zu erhalten. Die Metabolitenbildung der fünf Substanzen wurde zeit- und konzentrationsabhängig in humanen und tierischen Lebermikrosomen (LM) sowie verschiedenen humanen CYP-Enzymen (Supersomes$^{TM}$) untersucht. Die Metaboliten wurden charakterisiert, identifiziert und synthetisiert, um als Referenzverbindungen für die Quantifizierung sowie für weitere mechanistische in vitro Untersuchungen zu dienen. Ferner wurden enzymkinetische Parameter bestimmt, um eine Extrapolation der Metabolitenbildung hin zu kleinen Substratkonzentrationen sowie die Beteiligung humaner CYPs an der Metabolitenbildung in humanen LM zu ermöglichen. Außerdem wurde die Bildung von DNA-Addukten in primären Rattenhepatozyten nach Inkubation mit ME, MIE sowie deren Metaboliten untersucht. Für alle Verbindungen konnten die folgenden fünf enzymatischen Reaktionen identifiziert werden: Hydroxylierung der Seitenkette, Oxidation dieser Alkohole, Epoxidierung der Seitenkette und Hydrolyse zu Diolen sowie $\it O$-Demethylierung. Als Hauptmetaboliten von ME, MIE, aA und gA wurden die jeweiligen Seitenkettenalkohole identifiziert, während bA hauptsächlich über Epoxide zu Diolen metabolisiert wurde. Die Ergebnisse belegen, dass ME, im Gegensatz zu MIE, über den gleichen Mechanismus wie Safrol und Estragol, auch in geringen, humanrelevanten Konzentrationen aktiviert werden kann. Für aA und vor allem bA scheint eine Aktivierung über Epoxide wahrscheinlicher, während es für gA keine Hinweise auf potentiell gentoxische Metaboliten gibt.Methyleugenol (ME), ($\it E$)-methylisoeugenol (MIE), alpha-asarone (aA), beta-asarone (bA) and gamma-asarone (gA) are naturally occurring plant constituents, and thus part of the human nutrition. They belong to the class of phenylpropenes (PP) or alkenylbenzenes. ME, aA and bA are hepatocarcinogenic to male mice, whereas the carcinogenicity of MIE and gA were not investigated so far. Estragole and safrole, both allylic hepatocarcinogens, are activated via Cytochrom P450 enzymes (CYP) and sulfotransferases during xenobiotic metabolism by forming a 1’-hydroxy metabolite (side chain), which is afterwards sulfonated. After loss of sulfate, the remaining carbocation can form DNA-adducts leading to mutations and tumors in rodent liver. For ME, the same mechanism of activation was assumed, although ultimate evidences were lacking. In contrast, the majority of propenylic PP examined as yet is not or very weakly carcinogenic. The propenylic compounds aA and bA are hence an exception. Based on the results of animal experiments, an alternative mechanism of bioactivation is likely for aA and bA. Data on the metabolism of asarone-isomers are not yet available. Therefore, the aim of this work was to elucidate the metabolism of these five PP to uncover their possible carcinogenic mechanisms and metabolic pathways. The formation of metabolites was measured time- and concentration-dependent using human and animal liver microsomes, as well as different human CYP enzymes (Supersomes$^{TM}$). The metabolites were characterized, identified and synthesized to use them as reference compounds for quantification and to test them in further mechanistic in vitro assays. Kinetic data were derived to enable extrapolation to low substrate concentration, more relevant for the human nutrition, and to determine the contribution of different human CYPs in metabolite formation in human liver microsomes. Finally, the formation of DNA adducts by ME, MIE and there metabolites was studied using primary rat hepatocytes. For all compounds, the following five enzyme-catalyzed reactions were identified: hydroxylation of the side chain, oxidation of those alcohols to aldehydes or ketones, epoxidation of the side chain followed by hydrolysis yielding diols, as well as $\it O$-demethylation. Major metabolites of ME, MIE, aA and gA were the corresponding side chain alcohols, whereas bA was mainly metabolized via the epoxide/diol pathway. The results show, that ME, but not MIE, are bioactivated by the same mechanism as safrole and estragole even at low substrate concentrations. In case of aA and bA an alternative bioactivation via epoxides, not side chain alcohols, is highly likely. For gA, there are no hints for a genotoxic mode of action or genotoxic metabolites

Metabolism of the phenylpropanoids methyleugenol, (E\it E )-methylisoeugenol, alpha-, beta- und gamma-asarone

Methyleugenol (ME), ($\it E$)-Methylisoeugenol (MIE), alpha-Asaron (aA), beta-Asaron (bA) und gamma-Asaron (gA) sind natürlich vorkommende Pflanzeninhaltsstoffe aus der Klasse der Phenylpropene (PP) und Bestandteil der menschlichen Ernährung. ME, aA und bA erwiesen sich im Tierversuch als hepatokanzerogen. MIE und gA wurden bislang nicht untersucht. Die allylischen Hepatokanzerogene Estragol und Safrol werden im Verlauf des Fremdstoff-metabolismus durch Cytochrom P450-Enzyme (CYP) in 1‘-Position hydroxyliert und anschließend durch Sulfotransferasen sulfoniert. Der reaktive Schwefelsäureester kann DNA-Addukte bilden, wodurch in der Folge Mutationen und Tumoren ausgelöst werden können. Für ME wurde der gleiche Mechanismus der Aktivierung postuliert, obgleich definitive Beweise bislang fehlten. Im Gegensatz dazu war die Mehrheit der bislang untersuchten propenylischen PP nicht kanzerogen. aA und bA stellen insofern eine Ausnahme dar. Tierexperimentelle Untersuchungen legen einen alternativen Mechanismus für die Aktivierung von aA und bA nahe. Untersuchungen zum Metabolismus der Asarone liegen bisher nicht vor. Ziel der Arbeit war es daher, den hepatischen Metabolismus dieser fünf PP zu untersuchen, um Einblicke in die potentiellen gentoxischen Mechanismen zu erhalten. Die Metabolitenbildung der fünf Substanzen wurde zeit- und konzentrationsabhängig in humanen und tierischen Lebermikrosomen (LM) sowie verschiedenen humanen CYP-Enzymen (Supersomes$^{TM}$) untersucht. Die Metaboliten wurden charakterisiert, identifiziert und synthetisiert, um als Referenzverbindungen für die Quantifizierung sowie für weitere mechanistische in vitro Untersuchungen zu dienen. Ferner wurden enzymkinetische Parameter bestimmt, um eine Extrapolation der Metabolitenbildung hin zu kleinen Substratkonzentrationen sowie die Beteiligung humaner CYPs an der Metabolitenbildung in humanen LM zu ermöglichen. Außerdem wurde die Bildung von DNA-Addukten in primären Rattenhepatozyten nach Inkubation mit ME, MIE sowie deren Metaboliten untersucht. Für alle Verbindungen konnten die folgenden fünf enzymatischen Reaktionen identifiziert werden: Hydroxylierung der Seitenkette, Oxidation dieser Alkohole, Epoxidierung der Seitenkette und Hydrolyse zu Diolen sowie $\it O$-Demethylierung. Als Hauptmetaboliten von ME, MIE, aA und gA wurden die jeweiligen Seitenkettenalkohole identifiziert, während bA hauptsächlich über Epoxide zu Diolen metabolisiert wurde. Die Ergebnisse belegen, dass ME, im Gegensatz zu MIE, über den gleichen Mechanismus wie Safrol und Estragol, auch in geringen, humanrelevanten Konzentrationen aktiviert werden kann. Für aA und vor allem bA scheint eine Aktivierung über Epoxide wahrscheinlicher, während es für gA keine Hinweise auf potentiell gentoxische Metaboliten gibt.Methyleugenol (ME), ($\it E$)-methylisoeugenol (MIE), alpha-asarone (aA), beta-asarone (bA) and gamma-asarone (gA) are naturally occurring plant constituents, and thus part of the human nutrition. They belong to the class of phenylpropenes (PP) or alkenylbenzenes. ME, aA and bA are hepatocarcinogenic to male mice, whereas the carcinogenicity of MIE and gA were not investigated so far. Estragole and safrole, both allylic hepatocarcinogens, are activated via Cytochrom P450 enzymes (CYP) and sulfotransferases during xenobiotic metabolism by forming a 1’-hydroxy metabolite (side chain), which is afterwards sulfonated. After loss of sulfate, the remaining carbocation can form DNA-adducts leading to mutations and tumors in rodent liver. For ME, the same mechanism of activation was assumed, although ultimate evidences were lacking. In contrast, the majority of propenylic PP examined as yet is not or very weakly carcinogenic. The propenylic compounds aA and bA are hence an exception. Based on the results of animal experiments, an alternative mechanism of bioactivation is likely for aA and bA. Data on the metabolism of asarone-isomers are not yet available. Therefore, the aim of this work was to elucidate the metabolism of these five PP to uncover their possible carcinogenic mechanisms and metabolic pathways. The formation of metabolites was measured time- and concentration-dependent using human and animal liver microsomes, as well as different human CYP enzymes (Supersomes$^{TM}$). The metabolites were characterized, identified and synthesized to use them as reference compounds for quantification and to test them in further mechanistic in vitro assays. Kinetic data were derived to enable extrapolation to low substrate concentration, more relevant for the human nutrition, and to determine the contribution of different human CYPs in metabolite formation in human liver microsomes. Finally, the formation of DNA adducts by ME, MIE and there metabolites was studied using primary rat hepatocytes. For all compounds, the following five enzyme-catalyzed reactions were identified: hydroxylation of the side chain, oxidation of those alcohols to aldehydes or ketones, epoxidation of the side chain followed by hydrolysis yielding diols, as well as $\it O$-demethylation. Major metabolites of ME, MIE, aA and gA were the corresponding side chain alcohols, whereas bA was mainly metabolized via the epoxide/diol pathway. The results show, that ME, but not MIE, are bioactivated by the same mechanism as safrole and estragole even at low substrate concentrations. In case of aA and bA an alternative bioactivation via epoxides, not side chain alcohols, is highly likely. For gA, there are no hints for a genotoxic mode of action or genotoxic metabolites

Assessment and Characterization of DNA Adducts Produced by Alkenylbenzenes in Fetal Turkey and Chicken Livers

Formation of DNA adducts by five alkenylbenzenes, safrole, methyl eugenol, eugenol, and asarone with either alpha- or beta-conformation, was analyzed in fetal avian livers in two in ovo models. DNA reactivity of the carcinogens safrole and methyl eugenol was previously demonstrated in the turkey egg model, whereas non-genotoxic eugenol was negative. In the current study, alkenylbenzenes were also tested in the chicken egg model. Injections with alkenylbenzenes were administered to fertilized turkey or chicken eggs for three consecutive days. Three hours after the last injection, liver samples were evaluated for DNA adduct formation using the (32)P-nucleotide postlabeling assay. DNA samples from turkey livers were also analyzed for adducts using mass spectrometry. In both species, genotoxic alkenylbenzenes safrole, methyl eugenol, alpha- and beta-asarone produced DNA adducts, the presence and nature of which, with exception of safrole, were confirmed by mass spectrometry, validating the sensitivity of the (32)P-postlabeling assay. Overall, the results of testing were congruent between fetal turkey and chicken livers, confirming that these organisms can be used interchangeably. Moreover, data obtained in both models is comparable to genotoxicity findings in other species, supporting the usefulness of avian models for the assessment of genotoxicity as a potential alternative to animal models

Hepatic Metabolism of Carcinogenic β‑Asarone

β-Asarone (<b>1</b>) belongs to the group of naturally occurring phenylpropenes like eugenol or anethole. Compound <b>1</b> is found in several plants, e.g., <i>Acorus calamus</i> or <i>Asarum europaeum</i>. Compound <b>1</b>-containing plant materials and essential oils thereof are used to flavor foods and alcoholic beverages and as ingredients of many drugs in traditional phytomedicines. Although <b>1</b> has been claimed to have several positive pharmacological effects, it was found to be genotoxic and carcinogenic in rodents (liver and small intestine). The mechanism of action of carcinogenic <i><b>allylic</b></i> phenylpropenes consists of the metabolic activation via cytochrome P450 enzymes and sulfotransferases. <i>In vivo</i> experiments suggested that this pathway does not play a major role in the carcinogenicity of the <i><b>propenylic</b></i> compound <b>1</b> as is the case for other propenylic compounds, e.g., anethole. Since the metabolic pathways of <b>1</b> have not been investigated and its carcinogenic mode of action is unknown, we investigated the metabolism of <b>1</b> in liver microsomes of rats, bovines, porcines, and humans using ¹H NMR, HPLC-DAD, and LC-ESI-MS/MS techniques. We synthesized the majority of identified metabolites which were used as reference compounds for the quantification and final verification of metabolites. Microsomal epoxidation of the side chain of <b>1</b> presumably yielded (<i>Z</i>)-asarone-1′,2′-epoxide (<b>8a</b>) which instantly was hydrolyzed to the corresponding <i>erythro</i>- and <i>threo</i>-configurated diols (<b>9b</b>, <b>9a</b>) and the ketone 2,4,5-trimethoxyphenylacetone (<b>13</b>). This was the main metabolic pathway in the metabolism of <b>1</b> in all investigated liver microsomes. Hydroxylation of the side chain of <b>1</b> led to the formation of three alcohols at total yields of less than 30%: 1′-hydroxyasarone (<b>2</b>), (<i>E</i>)- and (<i>Z</i>)-3′-hydroxyasarone (<b>4</b> and <b>6</b>), with <b>6</b> being the mainly formed alcohol and <b>2</b> being detectable only in liver microsomes of Aroclor 1254-pretreated rats. Small amounts of <b>4</b> and <b>6</b> were further oxidized to the corresponding carbonyl compounds (<i>E</i>)- and (<i>Z</i>)-3′-oxoasarone (<b>5</b>, <b>7</b>). 1′-Oxoasarone (<b>3</b>) was probably also formed in incubations with <b>1</b> but was not detectable, possibly due to its rapid reaction with nucleophiles. Eventually, three mono-<i>O</i>-demethylated metabolites of <b>1</b> were detected in minor concentrations. The time course of metabolite formation and determined kinetic parameters show little species-specific differences in the microsomal metabolism of <b>1</b>. Furthermore, the kinetic parameters imply a very low dependence of the pattern of metabolite formation from substrate concentration. In human liver microsomes, 71–75% of <b>1</b> will be metabolized via epoxidation, 21–15% via hydroxylation (and further oxidation), and 8–10% via demethylation at lower as well as higher concentrations of <b>1</b>, respectively (relative values). On the basis of our results, we hypothesize that the genotoxic epoxides of <b>1</b> are the ultimate carcinogens formed from <b>1</b>

Salivary nitrate/nitrite and acetaldehyde in humans : potential combination effects in the upper gastrointestinal tract and possible consequences for the in vivo formation of N-nitroso compounds-a hypothesis

Subsequent to the dietary uptake of nitrate/nitrite in combination with acetaldehyde/ethanol, combination effects resulting from the sustained endogenous exposure to nitrite and acetaldehyde may be expected. This may imply locoregional effects in the upper gastrointestinal tract as well as systemic effects, such as a potential influence on endogenous formation of N-nitroso compounds (NOC). Salivary concentrations of the individual components nitrate and nitrite and acetaldehyde are known to rise after ingestion, absorption and systemic distribution, thereby reflecting their respective plasma kinetics and parallel secretion through the salivary glands as well as the microbial/enzymatic metabolism in the oral cavity. Salivary excretion may also occur with certain drug molecules and food constituents and their metabolites. Therefore, putative combination effects in the oral cavity and the upper digestive tract may occur, but this has remained largely unexplored up to now. In this Guest Editorial, published evidence on exposure levels and biokinetics of nitrate/nitrite/NOx, NOC and acetaldehyde in the organism is reviewed and knowledge gaps concerning combination effects are identified. Research is suggested to be initiated to study the related unresolved issues.publishe

Comparison of points of departure between subchronic and chronic toxicity studies on food additives, food contaminants and natural food constituents

In the past, it was generally accepted as a default assumption that No-Observed-Adverse-Effect Levels (NOAELs) or Lowest-Observed-Adverse-Effect Levels (LOAELs) in long-term toxicity studies are lower than in short-term ones, i.e. the toxic potency increases with prolonged exposure duration. Recent studies on pesticides and industrial chemicals reported that subacute, subchronic or chronic NOAELs/LOAELs are similar when study design factors are appropriately considered. We investigated whether these findings also apply to certain food constituents. After reviewing subchronic and chronic toxicity studies on more than 100 compounds, a total of 32 compounds could be included in the analysis. Geometric mean (GM) values of subchronic vs. chronic NOAEL or LOAEL ratios ranged from 1.0 to 2.0, with a geometric standard deviation from 2.2 to 4.2, which is consistent with data reported in the literature. While for many of the investigated compounds the ratio is around 1 - suggesting that health-based guidance values could appropriately be derived from subchronic toxicity studies - our study also identified some substances with higher ratios leading to a GM of around 2. The EFSA Scientific Committee suggested to apply an uncertainty factor of 2 to extrapolate from subchronic to chronic studies and, as a precautionary approach, we concur with this suggestion.publishe

Opinion on acetaldehyde as a flavouring substance : considerations for risk assessment

Acetaldehyd kommt natürlicherweise in zahlreichen Lebensmitteln vor und wird aufgrund seines fruchtigen Aromas auch als Aromastoff eingesetzt. Die Internationale Agentur für Krebsforschung (IARC) stufte Acetaldehyd als möglicherweise krebserregend sowie in Verbindung mit der oralen Aufnahme über alkoholhaltige Getränke als humanes Kanzerogen ein. Vor diesem Hintergrund stellt sich die Frage, ob die Verwendung von Acetaldehyd als Aromastoff weiterhin vertretbar ist. Die Senatskommission zur gesundheitlichen Bewertung von Lebensmitteln (SKLM) der Deutschen Forschungsgemeinschaft (DFG) hat die aktuelle Datenlage zur Bewertung des gesundheitlichen Risikos der Verwendung von Acetaldehyd als Aromastoff geprüft und hierzu eine Stellungnahme verabschiedet. Demnach kann die Frage, ob Acetaldehyd nach oraler Exposition in vivo gentoxisch und mutagen wirkt, derzeit nicht abschließend beantwortet werden. Weiterhin ist auch unklar, welchen Beitrag die Verwendung von Acetaldehyd als Aromastoff zur Gesamtexposition des Verbrauchers gegenüber Acetaldehyd leistet. Eine wissenschaftliche Bewertung des gesundheitlichen Risikos der Verwendung von Acetaldehyd als Aromastoff ist daher weiterhin nicht möglich. Die SKLM weist darauf hin, dass aufgrund des gentoxischen Gefährdungspotenzials sowie zahlreicher Datenlücken, die für eine vollständige Risikobewertung geschlossen werden müssen, Zweifel an der Sicherheit von Acetaldehyd als Aromastoff bestehen. Nach Ansicht der SKLM sollte der gezielte Zusatz von Acetaldehyd als Aromastoff aus Gründen des vorsorgenden Verbraucherschutzes neu beurteilt werden.publishe

Evaluation of the genotoxic potential of acrylamide: Arguments for the derivation of a tolerable daily intake (TDI value)

This opinion of the Senate Commission on Food Safety (SKLM) of the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) presents arguments for an updated risk assessment of diet-related exposure to acrylamide (AA), based on a critical review of scientific evidence relevant to low dose exposure. The SKLM arrives at the conclusion that as long as an appropriate exposure limit for AA is not exceeded, genotoxic effects resulting in carcinogenicity are unlikely to occur. Based on the totality of the evidence, the SKLM considers it scientifically justified to derive a tolerable daily intake (TDI) as a health-based guidance value