Alternative methanogenesis - Methanogenic potential of organosulfur administration

Mammalian methanogenesis is principally linked to carbohydrate fermentation by the anaerobic intestinal microflora [1], but alternative pathways for non-bacterial methane (CH4) generation have also been described in aerobic living systems [2, 3]. In this respect it has been shown that aerobic CH4 can be readily formed from organosulfur compounds at ambient atmospheric pressure and temperature in vitro [4]. When using iron (II/III), hydrogen peroxide and ascorbic acid as reagents, the S-methyl groups of methionine and other organosulfur compounds are efficiently converted into CH4 [4]. To-date in vitro and in vivo data have established the possibility of biotic, non-archaeal CH4 generation under various hypoxia-linked conditions as well [5–7]. An important novel aspect of this process is CH4 bioactivity [6]. While CH4 is conventionally believed to be physiologically inert, anti-inflammatory effects were described for exogenous CH4 in several experimental hypoxia-reoxygenation conditions [8]. Therefore, these and other data collectively raised the possibility that non-bacterial CH4 emissions can be part of an adaptive response to oxido-reductive stress in eukaryotes [6, 7]. Based on this background we hypothesized that the metabolic supply of molecules that contain the sulfhydryl (SH) side chain can influence the biotic methanogenesis. A promising candidate appeared to be allyl isothiocyanate, a naturally occurring biothiol compound, particularly abundant in mustard seed [9]. Biothiols can easily be internalized via dietary intake [10], and theoretically, this route might influence the process of CH4 formation. Therefore, the major aim of our study was to investigate the methanogenic potential of organosulfur compounds in model experimental systems of oxido-reductive stress. In this line, the impact of increased biothiol intake (organosulfur-enriched or SH-diet) was examined in association with established markers of redox imbalance in a rodent model of hepatic dysfunction induced by high oral doses of ethanol.

Sulfur-containing test compounds included dimethyl-thiourea (DMTU), 2-mercaptoethanolsulfonic acid (MES), aqueous solutions of white mustard seed (Brassica hirta) and coriander seed (Coriandrum sativum) extracts and dimethyl-sulfoxide (DMSO) as positive control [4,5]. All chemicals were purchased from Sigma-Aldrich (Merck, Budapest, Hungary), commercial plant seed products (200-200 mg) from Kotanyi Ltd. were extracted with water (10 mL). The reaction mixture was adjusted to pH 7.4, the tubes were then closed the reaction was initiated by the addition of H 2 O 2 with a Hamilton syringe through the septum of the cap.
The emanating gas was detected in real-time by photoacoustic spectroscopy (PAS) for 10 min as described previously [11]. Briefly, PAS is a special mode of spectroscopy which measures optical absorption indirectly via the conversion of absorbed light energy into acoustic waves. The amplitude of the generated sound is directly proportional to the concentration of the absorbing gas component. The light source of the system is a near-infrared diode laser that emits around the CH 4 absorption line at 1650.9 nm with an output power of 15 mW (NTT Electronics, Tokyo, Japan). Cross-sensitivity for common components of breath and ambient air were repeatedly examined, and no measurable instrument response was found for several vol % of CO 2 or H 2 O vapor. The narrow line width of the diode laser provides high selectivity; the absorbance of CH 4 is several orders of magnitude greater than that of H 2 O, CO 2 or CO at 1.65 μm, the wavelength we used. The device was previously calibrated with various gas mixtures prepared by dilution of 100 ppm CH 4 in synthetic air (Messer, Budapest, Hungary), and it has a dynamic range of 4 orders of magnitude; the minimum detectable concentration of the sensor was found to be 0.25 ppm (3σ), with an integration time of 12 s.

In vivo experiments
The animal experiments were performed on a total of 49 male SKH-1 hairless mice (weighing 30-36 g) in accordance with the National Institutes of Health guidelines on the handling and care of experimental animals and EU Directive 2010/63 for the protection of animals used for scientific purposes. The study protocol was reviewed by the National Scientific Ethical Committee on Animal Experimentation (National Competent Authority of Hungary) and was approved by the Animal Welfare Committee of the University of Szeged (approval No. V/148/ 2013 and I-74-16/2018).

Experimental protocols
Whole-body CH 4 measurements were carried out (see details later) and animals with non-producer CH 4 status (less than 0.2 ppm CH 4 changes in comparison to the ambient air) were included. After this pre-screening, the non-CH 4 producer animals were randomly allocated into 7 experimental groups. Group 1 (n = 7) served as control, these mice were fed with standard laboratory chow (Innovo Kft., Isaszeg, Hungary) for 7 days. In Groups 2 and 3 (n = 7, each) the animals received standard laboratory chow enriched with 10% mustard-seed for 7 days or 14 days, respectively.
Large amounts of oral ethanol induce methanogenesis in non-CH 4 producer humans and rodents [7], thus in the second part of the in vivo study the effect of SH-diet combined with ethanol challenge was monitored. These mice were fed with standard diet for 7 days (Groups 4 and 5; n = 7) or 10% mustard-seed containing chow for 7 or 14 days (Groups 6 and 7; n = 7, each). After this period the animals were provided drinking water (Group 4, n = 7) or water containing 12% ethanol (Groups 5-7) for further 7 days while fed with standard laboratory chow. Ad libitum supply of the food and water, or ethanol solution was provided and the daily consumption was recorded.
The whole-body CH 4 emission of mice was measured using the PAS system in a specially designed closed glass sampling chamber with an internal volume of 180 cm 3 as reported previously [11]. Prior to the experiments the CH 4 concentration in the chamber was determined and used as baseline value. The mouse was placed into the chamber which was then sealed and a sample of the chamber gas was analyzed exactly 20 min later. The mouse was then removed, and the measurements were repeated on the subsequent days. The chamber was thoroughly flushed with room air before the next animal was placed in. The whole-body CH 4 emission was calculated as the difference in the baseline CH 4 concentration (using 50 measuring point area under the curve) and the cumulative CH 4 released during the first 5 min of the 20-min sample collection period (using 50 measuring point area under the curve) and then referred to the body weight.

Determination of the total thiols and non-protein bound thiols in liver samples
After the last gas measurements, the animals were anaesthetized (90 mg/kg ketamine + 25 mg/kg xylazine ip), and tissue biopsies were subsequently taken from the liver. At the end of sample taking, the animals were euthanized by overdosing the anesthetics (300 mg/kg ketamine + 30 mg/kg xylazine ip). After measuring the wet weight, total sulfhydryl groups (TSH) in tissue homogenates (applying Potter homogenizer in PBS-based homogenization buffer containing 0.25 M sucrose, 10 mM Tris, 0.5 mM EDTA at pH 7.4) were spectrophotometrically determined (UV-1601 spectrophotometer, Shimadzu, Kyoto, Japan) by the method of Ellman [12] as well as reduced glutathione content (GSH) in using the method and calculation of Sedlak and Lindsay [13] and the concentrations were referred to tissue wet weight.

Determination of GSH/GSSG ratio in liver tissue samples
The GSH/GSSG ratio was detected using a Sigma-Aldrich (Merck, Budapest, Hungary, catalogue No. 38185-1KT) kit according to the manufacturers instruction at 420 nm using a platereader (Fluorostar Omega, BMG Labtech, Ortenberg, Germany). Calculation was made by using the standard calibration curve and values were normalized to wet tissue weight.

Determination of NADPH oxidase (NOX) activity in liver biopsies
The liver samples were homogenized then 50 μl of re-suspended homogenate was added in Dulbecco's solution containing lucigenin (10 mM), EGTA (10 mM) and sucrose (900 mM). The NADPH oxidase activity was determined via the NADPH-dependent increase in luminescence elicited by adding 1 mM NADPH (in 20 μl), measured with an FB12 Single Tube Luminometer (Berthold Detection Systems GmbH, Bad Wildbad, Germany). Samples incubated in the presence of nitroblue-tetrazolium served as controls. The protein content of the samples was determined with Lowry's method. The measurements were performed in triplicates and were normalized for protein content.

Statistical analysis
Data analysis was performed with the statistical software package GraphPad Prism 5.01 for Windows (GraphPad Software, La Jolla, California, USA). For time-dependent differences within groups, repeated measures Kruskal-Wallis test with Dunn's multiple comparison test, and for time-dependent differences of multiple groups repeated measures two-way ANOVA on ranks, followed by Bonferroni post-hoc test was applied. Kruskal-Wallis test with Bonferroni post-hoc test was applied to compare multiple groups. In the figures, mean values and standard deviation (SD) are given, p values < 0.05 were considered significant.

In vitro CH 4 emission
The relative effectiveness of different organosulfur compounds in terms of methanogenesis was detected in a chemical model reaction (Fig 1). DMSO is a potent CH 4 donor molecule in the presence of reactive oxygen species (ROS) generation [4,5] Methanogenesis in vitro was promptly induced in the DMSO-containing tubes (10925±3139 ppm) while lower CH 4 emission was detected in the DMSO+DMTU-containing tubes (6954±784 ppm; p = 0.164 vs DMSO). MES, however, augmented the methanogenic capacity of DMSO (12833±1666 ppm; p = >0.9999 vs DMSO). A significant increase was detected after the administration of mustard seed extract (14979±2812 ppm; p = <0.0001 vs DMSO+DMTU), while the released CH 4 amount was lower in the coriander seed extract-containing tubes as compared to mustard seed

PLOS ONE
extract-induced generation (9364±872 ppm; p = 0.0294). Specifically, the mustard-seed extract kept up the process of methanogenesis during the 10-min observation period.

CH 4 release in mice exposed to ethanol challenge
The baseline CH 4 values did not differ significantly and did not exceed 1547 PAU/kg -1 in any of the animals on the night before the ethanol feeding protocol was started (Fig 3A). The dietary ethanol challenge led to a significant CH 4 release in all groups in comparison to control (Fig 3B). 24h of ethanol consumption was already resulted in slightly elevated CH 4 output in the ethanol-fed group compared to the control group (day 1: Group 5: 1070±645 PAU/kg -1 vs 397±276PAU/kg -1 ) and the whole-body CH 4 was significantly higher in those groups which were fed with the SH-diet (day 1: Group 6: 1864±1079 PAU/kg -1 ; Group 7: 2042±798PAU/kg -1 ). This pattern was sustained until the 5th day of the ethanol-feeding protocol. Thereafter the highest CH 4 emanation was detected in animals with no antecedent SH-diet (day 5: Group 5: 3242±693PAU/kg -1 vs Group 7: 2492±556 PAU/kg -1 ) and this was sustained thereafter (day 7: Group 5: 5073±406 PAU/kg -1 ; Group 6: 4089±833PAU/kg -1 ; Group 7: 3540±654 PAU/kg -1 ). No changes were observed in the untreated control group during the observation period (day 7: 223±107 PAU/kg -1 ) as compared to those were measured on day 1.

Total thiols and non-protein bound thiols in liver samples of the healthy, unexposed mice
The liver GSH content did not change in the SH-diet groups as compared to the control group (( Fig 4A); Group 2: 8.83±1.49 μmol/mL/100 mg tissue vs Group 3: 10.3±1.62 μmol/mL/100 mg tissue vs Group 1: 11.61±4.51 μmol/mL/100 mg tissue). Nevertheless, the overall thiol concentration in the liver significantly increased by the SH-diet in both groups (( Fig 4B); Group 2: 66.32±13.47 μmol/mL/100 mg tissue vs Group 3: 69.24±10.07 μmol/mL/100 mg tissue vs. Group 1: 48.86±7.924 μmol/mL/100 mg tissue). In line with these data, the total protein concentration of the samples in Groups 2 and 3 also increased (control vs 2-weeks SH-diet p = 0.0441, data not shown) which might also reflect alterations in the SH-containing amino acid metabolism caused by the diet.

Discussion
SH-containing amino acids and their metabolites can be involved in the process of in vitro, aerobic methanogenesis [4]. Taking these findings into consideration we hypothesized that mustard seed extract could act as a potential source of non-microbial CH 4 formation. After confirming this possibility, in vivo experiments were performed with dietary SH-enrichment in non-CH 4 producer rodents with or without ethanol stress. Changes in liver thiol concentrations and representative oxido-reductive parameters were detected in order to shed light on the mechanistic aspects of the methanogenic reaction. On the one hand, the concentration of thiol-containing molecules in the liver was significantly increased, suggesting an influence on biothiol tissue storage. On the other hand, the SH-diet per se did not influence the net CH 4 output which might suggested that the onset of oxido-reductive imbalance is needed for an amplified CH 4 release. As expected, when these animals were involved in the high-dose ethanol-feeding protocol, methanogenesis was induced. The SH-diet remarkably increased the CH 4 release after 24h of ethanol consumption, and this effect was not observed in animals kept on standard feeding. What is more, the CH 4 production declined in the SH-enriched diet groups by the end of the ethanol protocol possibly due to pertaining tissue protective events.
In contrast, in groups with no extra biothiol feeding, the highest CH 4 production was reached by the later time point. The time course of the methanogenic reaction was somewhat different to that observed in a previous in vivo study [7] but this can be explained by interspecies differences to ethanol-induced adaptation as demonstrated recently by a large-scale work of Adkins et al. on cross-species comparisons of ethanol-responsive genes [14].
One of the consequences of alcohol consumption is NADH+H + production which leads to reductive stress and a critical disruption of the cellular redox balance [15]. Althoff and coworkers demonstrated that non-microbial methanogenesis can take place in a biphasic oxidation/sulfoxidation of methyl sulfides, plausible only in highly reductive environment [4]. As sulfoxidation of methyl sulfides is ubiquitous in the environment in the presence of high level of ROS, this chemical route is most likely to facilitate CH 4 release in living aerobic organisms. This concept can explain the ethanol-induced CH 4 generation, but further experimental data are clearly needed to shed more light on the underlying organosulfur biochemistry. It should be added that NOX is the major source of ROS and play predominant roles in the pathogenesis of early alcohol-induced hepatic injury [16]. More importantly, the liver NOX activity was significantly elevated after the ethanol challenge; however, it was moderated in the SH-diet groups.
To-date many studies have examined the effects of high intakes of SH amino acids (i.e. methionine and cysteine) on growth and life cycle in rodents [17]. A limited number of studies have also been conducted on the impact on inflammatory responses [18] and the in vitro effects of metabolic products of SH-amino acids on human immune cells [19]. Cellular concentrations of biothiols and GSH have been linked to T-cell numbers [20]; the tissue biothiol content responds to SH-amino acid intake by a broad range of intra/intercellular signaling pathways involving metabolization of the essential cofactors leading to GSH production (e.g. vitamin B6, riboflavin, and folic acid). Administration of thiol-containing GSH precursors as n-acetyl cysteine (NAC) or L-2-oxothiazolidine-4-carboxylate (OTC), are of interest in nutrition research [21]. It has been shown that a low-protein diet would suppress GSH synthesis, a situation that is reversed by cysteine or methionine [22]. Despite these supportive data, the impact of a high SH-amino acid diet on established inflammatory responses has not been investigated in any depth in humans or experimental animals.
The anti-inflammatory potential of organosulfur-containing plants such as mustard, horseradish or wasabi is usually linked to the AITC content of these herbs [9,23] and it has been demonstrated that the nuclear factor erythroid 2-related factor 2 (Nrf2) and NF-κB routes are involved in the signaling pathways [23]. Several lines of evidence indicate that Nrf2 plays a key role in the regulation of cellular SH-amino acid/GSH turnover and controlled GSH/GSSG ratio as well. In vitro, Nrf2 regulates the cysteine/glutamate exchange transporter that maintains intracellular GSH levels by allowing cysteine influx [24]. Besides, Nrf2 regulates glutathione peroxidase (GPX2) and glutathione-S-transferase (GST) in human alveolar epithelial cells [25]. In addition, higher levels of GSSG was detected in Nrf2−/− alveolar macrophages than in wild-type Nrf-2 cells in response to oxido-reductive stress stimuli [26]. In the current study, we demonstrated a significant contribution of SH-diet on liver biothiol storage as the quantity of molecules with SH residues elevated significantly after 7 days of feeding. Interestingly, the tissue non-protein-bound thiol content (which is represented in 95% by GSH) did not alter in a same manner although the total protein concentration correlated positively with the changes of TSH, indicating that the liver antioxidant capacity was embedded to its proteome, rather than in free GSH. The SH-diet contained a relatively high concentration (10%) of crashed mustard-seed, thoroughly mixed and enriched into the standard laboratory chow which enabled ad libitum consumption. Others have already reported on the use of 5 to 10% mustard-seed enriched diet with anti-inflammatory and anti-carcinogenic effects in mice [27].
Upon the emerging reductive stress of the ethanol challenge, these biochemical routes were possibly activated to potentiate the metabolization of GSH from its amino-acid precursors. This possibility is supported by the elevated GSH/GSSG ratio in the liver samples in the conventional diet-fed group after the ethanol protocol, as a higher ratio of GSH to GSSG suggests a reductive environment [28]. This elevation was not present in the mustard-seed diet groups and the GSH/GSSG ratios remained at the control levels. The accumulation of GSH can be linked to ethanol-evoked over-expression of Nrf2 as reported in similar stress conditions [29]. Nevertheless, further studies are necessary to investigate the direct interactions between the inflammatory trigger, non-microbial methane generation, AITC concentration and Nfr2 and GST/GPX2 expression patterns.

Conclusion
In summary, the present study provides in vivo evidence for the possibility of an SH-enriched diet to trigger non-microbial methanogenesis during ethanol feeding. The potential benefit of this approach is to influence a condition associated with redox imbalance, it might be a reasonable choice in cases where tissue hypoxia-related complications are predicted.

S1 Fig. Protocols for the in vivo experiments.
In Phase I, the effects of SH-diet (laboratory chow enriched with 10% mustard-seed) without ethanol feeding was monitored. The animals in control Group 1 were fed with standard laboratory chow for 7 days. In Groups 2 and 3, the animals received SH-diet for 7 or 14 days, respectively. In Phase II, the effect of SH-diet combined with ethanol challenge was investigated. These mice were fed with standard laboratory chaw for 7 days (Groups 4 and 5) or SH-diet for 7 or 14 days (Groups 6 and 7). After this period, the animals were provided drinking water (Group 4) or water containing 12% ethanol (Groups 5-7) for further 7 days while fed with standard laboratory chow. (TIF) S2 Fig. Summary of findings. Administration of plant extracts containing organosulfur (SH) moieties can increase the thiol content of the liver without influencing baseline methanogenesis. Ethanol consumption induces hepatic oxido-reductive-stress condition, which is linked to increased methanogenesis, most probably a potentially tissue-protective mechanism. Preceding oral SH feeding can effectively reduce the ethanol-induced hepatic injury. (TIFF)