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Abstract

Ergothioneine is a histidine thio-derivative isolated in 1909. In ergothioneine biosynthesis, the combination of a mononuclear non-heme iron enzyme catalyzed oxidative C-S bond formation reaction and a PLP-mediated C-S lyase (EgtE) reaction results in a net sulfur transfer from cysteine to histidine side-chain. This demonstrates a new sulfur transfer strategy in the biosynthesis of sulfur-containing natural products. Due to difficulties associated with the overexpression of Mycobacterium smegmatis EgtE protein, the proposed EgtE functionality remained to be verified biochemically. In this study, we have successfully overexpressed and purified M. smegmatis EgtE enzyme and evaluated its activities under different in vitro conditions: C-S lyase reaction using either thioether or sulfoxide as a substrate in the presence or absence of reductants. Results from our biochemical characterizations support the assignment of sulfoxide 4 as the native EgtE substrate and the involvement of a sulfenic acid intermediate in the ergothioneine C-S lyase reaction.

Results

We sub-cloned M. smegmatis EgtE gene into the pASK-IBA3+ vector and the protein was overexpressed in the E. coli BL21(DE3) strain. The expressed EgtE was then purified using Strep-Tactin resin to near homogeneity (higher than 95% based on the analysis of the SDS-PAGE, ). Furthermore, as suggested by EgtE sequence analysis, its UV-visible spectra are consistent with the presence of a PLP cofactor (). The pH-dependence of the EgtE internal aldimine electronic absorption was also studied. In PLP-containing proteins, the PLP-Lysine conjugate can exist in a few different forms: enolimine (9), ketoenamine (10) or deprotonated ketoenamine (11),,,,,,,. For the isolated EgtE protein at pH 7.5, it has an absorption feature near 420 nm (), which is consistent with the presence of ketoenamine (10) tautomer of the protonated Schiff base. As the pH changes from 6.0 to 10.0, the UV-vis spectra () do not show distinct changes in the overall absorption features and the maximal absorption wavelengths. Thus, for EgtE, the internal aldimine is present mainly in the form of 10. The amount of PLP per EgtE monomer was also determined after it was released by 0.2 M NaOH treatment. The isolated EgtE has 0.8 mole PLP per mole of EgtE monomer.

Methods

To clarify the exposure of ERGO to brain and the possible antidepressant‐like effect after oral ingestion, ERGO or GOME (golden oyster mushroom extract) which contains 1.2% (w/w) ERGO was mixed with feed and provided to mice for 2 weeks, and then ERGO concentration and antidepressant‐like effect were evaluated by LC‐MS/MS and FST (forced swimming test) or TST (tail suspension test), respectively.

Results

Diet containing ERGO or GOME greatly increased the ERGO concentrations in plasma and brain, and significantly decreased the immobility time in both FST and TST. The required amount of GOME (~37 mg/day) to show the antidepressant‐like effect corresponds to at most 8 g/day in humans. In mice receiving GOME‐containing diet, doublecortin‐positive cells showed a significant increase from the basal level, suggesting promotion of neuronal differentiation.

Animals

C57BL/6J mice were purchased from Sankyo Labo Service Co. (Tokyo, Japan). About 5–8‐week‐old male mice were housed under pathogen‐free conditions at controlled temperature (21–25°C) with a 12 h light/dark cycle. The lights remained on from 8:00 to 20:00, and food and water were available ad libitum. The animals were cared for in strict compliance with the guidelines outlined in the National Institutes of Health Guide for the Care and Use of Laboratory Animals. All animal procedures used in this work were approved by the Kanazawa University Animal Care Committee.

Antidepressant‐like effect of ERGO

Next, we examined the antidepressant‐like effect of ERGO. GOME or ERGO was fed to mice in the diet for 2 weeks, and FST, which is a well‐established tool for assessing antidepressant‐like activity in rodents, was performed (Fig. B). The immobility time of mice given diet containing 10% GOME was significantly lower than that of mice given normal diet [F(5,70) = 2.9, P < 0.05] (Fig. A). To examine whether this activity of GOME is due to ERGO present in GOME, mice were fed diet containing authentic ERGO, the amount of which (120 mg ERGO/100 g diet) was equivalent to that in diet containing 10% GOME. The immobility time in mice given diet containing 10% GOME was similar to that given diet containing the equivalent amount of authentic ERGO (Fig. A). In addition, ERGO concentration in the brain in mice given diet containing 10% GOME was also similar to that given diet containing the equivalent amount of authentic ERGO (Fig. ). These data suggested that the antidepressant‐like effect of GOME is predominantly due to ERGO contained in the extract.

Abstract

Metabolomic analysis revealed that Methylobacterium cells accumulate a large amount of ergothioneine (EGT), which is a sulfur-containing, non-proteinogenic, antioxidative amino acid derived from histidine. EGT biosynthesis and its role in methylotrophy and physiology for plant surface-symbiotic Methylobacterium species were investigated in this study. Almost all Methylobacterium type strains can synthesize EGT. We selected one of the most productive strains (M. aquaticum strain 22A isolated from a moss), and investigated the feasibility of fermentative EGT production through optimization of the culture condition. Methanol as a carbon source served as the best substrate for production. The productivity reached up to 1000 μg/100 ml culture (1200 μg/g wet weight cells, 6.3 mg/g dry weight) in 38 days. Next, we identified the genes (egtBD) responsible for EGT synthesis, and generated a deletion mutant defective in EGT production. Compared to the wild type, the mutant showed better growth on methanol and on the plant surface as well as severe susceptibility to heat treatment and irradiation of ultraviolet (UV) and sunlight. These results suggested that EGT is not involved in methylotrophy, but is involved in their phyllospheric lifestyle fitness of the genus in natural conditions.