Metabolic engineering of the regulators in nitrogen catabolite repression to reduce the production of ethyl carbamate in a model rice wine system

High Salt-Resistant Urethanase Degrades Ethyl Carbamate in Soy Sauce

Urethanase is a promising biocatalyst for degrading carcinogen ethyl carbamate (EC) in fermented foods. However, their vulnerability to high ethanol and/or salt and acidic conditions severely limits their applications. In this study, a novel urethanase from Alicyclobacillus pomorum (ApUH) was successfully discovered using a database search. ApUH shares 49.4% sequence identity with the reported amino acid sequences. It belongs to the Amidase Signature family and has a conserved "K-S-S" catalytic triad and the characteristic "GGSS" motif. The purified enzyme overexpressed in Escherichia coli exhibits a high EC affinity (Km, 0.306 mM) and broad pH tolerance (pH 4.0-9.0), with an optimum pH 7.0. Enzyme activity remained at 58% in 12% (w/v) NaCl, and 80% in 10% (v/v) ethanol or after 1 h treatment with the same ethanol solution at 37 °C. ApUH has no hydrolytic activity toward urea. Under 30 °C, the purified enzyme (200 U/L) degraded about 15.4 and 43.1% of the EC in soy sauce samples (pH 5.0, 6.0), respectively, in 5 h. Furthermore, the enzyme also showed high activity toward the class 2A carcinogen acrylamide in foods. These attractive properties indicate their potential applications in the food industry.

Quality and flavor development of solid-state fermented surimi with Actinomucor elegans: A perspective on the impacts of carbon and nitrogen sources

The influence of four carbon and nitrogen substrates on the quality and flavor of a novel surimi-based product fermented with Actinomucor elegans (A. elegans) was investigated, with a focus on carbon and nitrogen catabolite repression. The results showed that the substrate significantly affected mycelial growth, enzyme activities, and the metabolites of A. elegans. Although glucose significantly promoted A. elegans growth by 116.69%, it decreased enzyme secretion by 69.79% for α-amylase and 59.80% for protease, most likely by triggering the carbon catabolite repression pathway. Starch, soy protein, and wheat gluten substantially affected the textural properties of the fermented surimi. Furthermore, wheat gluten significantly promoted the protease activity (102.70%) and increased protein degradation during surimi fermentation. The fishy odor of surimi was alleviated through fermentation, and a correlation between the volatile compounds and A. elegans metabolism was observed. These results explore fermentation substrates in filamentous fungi metabolism from a catabolite repression perspective.

TORC1 Signaling in Fungi: From Yeasts to Filamentous Fungi

Target of rapamycin complex 1 (TORC1) is an important regulator of various signaling pathways. It can control cell growth and development by integrating multiple signals from amino acids, glucose, phosphate, growth factors, pressure, oxidation, and so on. In recent years, it has been reported that TORC1 is of great significance in regulating cytotoxicity, morphology, protein synthesis and degradation, nutrient absorption, and metabolism. In this review, we mainly discuss the upstream and downstream signaling pathways of TORC1 to reveal its role in fungi.

Draft Genome Sequence of Lactobacillus brevis Strain 2-34, Isolated from the Shaoxing Huangjiu Fermentation Process

Here, we announce the draft genome sequence of Lactobacillus brevis 2-34, a strain isolated from the fermentation process of Shaoxing huangjiu (Chinese rice wine). The genome size of 2-34 was 2,557,496 bp, with 2,459 coding genes, 67 tRNAs, and 2 rRNAs.

Occurrence of Ethyl Carbamate in Foods and Beverages: Review of the Formation Mechanisms, Advances in Analytical Methods, and Mitigation Strategies

ABSTRACT

Ethyl carbamate (EC) is a process contaminant that can be formed as a by-product during fermentation and processing of foods and beverages. Elevated EC concentrations are primarily associated with distilled spirits, but this compound has also been found at lower concentrations in foods and beverages, including breads, soy sauce, and wine. Evidence from animal studies suggests that EC is a probable human carcinogen. Consequently, several governmental institutions have established allowable limits for EC in the food supply. This review includes EC formation mechanisms, occurrence of EC in the food supply, and EC dietary exposure assessments. Current analytical methods used to detect EC will be covered, in addition to emerging technologies, such as nanosensors and surface-enhanced Raman spectroscopy. Various mitigation methods have been used to maintain EC concentrations below allowable limits, including distillation, enzymatic treatments, and genetic engineering of yeast. More research in this field is needed to refine mitigation strategies and develop methods to rapidly detect EC in the food supply.

HIGHLIGHTS

The Threat of Multiple Liver Carcinogens in the Population of Laos: A Review

Laos is a landlocked country in South East Asia, ranking fifth for primary liver cancer incidence worldwide. Risk factors that might explain this worrying situation are poorly known. We conducted a review of the literature concerning the etiologies of terminal liver diseases in Laos. A double infectious burden with hepatitis B and C viruses and the liver fluke Opisthorchis viverrini seems to be the main cause of the high liver cancer incidence. Moreover, it was also suggested that mutagenic substances frequently found in tobacco, alcoholic beverages, fermented fish, and mold-contaminated cereals or nuts, which are all substances heavily consumed by Lao people, lead to the accumulation of DNA mutations in the liver cell genome causing tumor processes. However, the respective proportions of liver cancer cases attributable to each category of infections and substances consumed, as well as the histological nature of the neoplasia are still not precisely documented in Laos. The international medical and scientific communities as well as public health stakeholders should urgently consider the alarming situation of liver health in Laos to stimulate both research and subsequent implementation of prevention policies.

Chinese Yellow Rice Wine Processing with Reduced Ethyl Carbamate Formation by Deleting Transcriptional Regulator Dal80p in Saccharomyces cerevisiae

Ethyl carbamate (EC) is a potential carcinogen that forms spontaneously during Chinese rice wine fermentation. The primary precursor for EC formation is urea, which originates from both external sources and arginine degradation. Urea degradation is suppressed by nitrogen catabolite repression (NCR) in Saccharomyces cerevisiae. The regulation of NCR is mediated by two positive regulators (Gln3p, Gat1p/Nil1p) and two negative regulators (Dal80p/Uga43p, Deh1p/Nil2p/GZF3p). DAL80 revealed higher transcriptional level when yeast cells were cultivated under nitrogen-limited conditions. In this study, when DAL80-deleted yeast cells were compared to wild-type BY4741 cells, less urea was accumulated, and genes involved in urea utilization were up-regulated. Furthermore, Chinese rice wine fermentation was conducted using dal80Δ cells; the concentrations of urea and EC were both reduced when compared to the BY4741 and traditional fermentation starter. The findings of this work indicated Dal80p is involved in EC formation possibly through regulating urea metabolism and may be used as the potential target for EC reduction.

Regulation and metabolic engineering strategies for permeases of Saccharomyces cerevisiae

Microorganisms have evolved permeases to incorporate various essential nutrients and exclude harmful products, which assists in adaptation to different environmental conditions for survival. As permeases are directly involved in the utilization of and regulatory response to nutrient sources, metabolic engineering of microbial permeases can predictably influence nutrient metabolism and regulation. In this mini-review, we have summarized the mechanisms underlying the general regulation of permeases, and the current advancements and future prospects of metabolic engineering strategies targeting the permeases in Saccharomyces cerevisiae. The different types of permeases and their regulatory mechanisms have been discussed. Furthermore, methods for metabolic engineering of permeases have been highlighted. Understanding the mechanisms via which permeases are meticulously regulated and engineered will not only facilitate research on regulation of global nutrition and yeast metabolic engineering, but can also provide important insights for future studies on the synthesis of valuable products and elimination of harmful substances in S. cerevisiae.

Molecular Engineering of Bacillus paralicheniformis Acid Urease To Degrade Urea and Ethyl Carbamate in Model Chinese Rice Wine

Bacillus paralicheniformis urease (BpUrease) has been shown to be a promising biocatalyst for degrading the carcinogenic chemical ethyl carbamate (EC or urethane) in rice wine. However, low EC affinity and catalytic efficiency limit the practical application of BpUrease. In this study, we improved the EC degradation capability of BpUrease by site-saturation mutagenesis (SSM). The best variant L253P/L287N showed a 49% increase in EC affinity, 1027% increase in catalytic efficiency ( k_cat/ K_m), and 583% increase in half-life ( t_1/2) at 70 °C. Homology modeling analysis suggest that mutation of Leu253 to Pro increased the BpUrease EC specificity by affecting the interaction between Arg339 with the catalytic residue His323, while Leu287Asn mutation benefits EC specificity and affinity by changing the interaction networks among the residues in the catalytic pocket. Our results show that the L253P/L287N variant efficiently degraded urea and EC in a model rice wine, making it a good candidate for practical application in the food industry.

Metabolic Engineering of Four GATA Factors to Reduce Urea and Ethyl Carbamate Formation in a Model Rice Wine System

Urea is the most important precursor of ethyl carbamate (EC), a harmful carcinogenic product, in fermented wines. In this study, the effects of four GATA transcriptional factors (Gln3p, Gat1p, Dal80p ,and Gzf3p) on extracellular urea and EC formation and transcriptional changes in urea degradation related genes ( DUR1,2 and DUR3) were examined. Compared to the WT strain, the Δ gzf3 mutant showed 18.7% urea reduction and exhibited synergistic effects with overexpressed Gln3p_1-653 and Gat1p_1-375 on extracellular urea reduction. Moreover, Δ gzf3+Gln3p_1-653 and Δ gzf3+Gat1p_1-375 showed significant 38.7% and 43.7% decreases in urea concentration and 41.7% and 48.5% decreases in EC concentration, respectively, in a model rice wine system. These results provide a promising way to reduce urea and EC formation during wine fermentation and raise some cues for the regulations of the four GATA transcriptional factors on the expression of individual nitrogen catabolite repression sensitive genes and their related metabolism pathway.

Adaptive Evolution Relieves Nitrogen Catabolite Repression and Decreases Urea Accumulation in Cultures of the Chinese Rice Wine Yeast Strain Saccharomyces cerevisiae XZ-11

Urea is the major precursor of ethyl carbamate in Chinese rice wine. Although efforts have been made to decrease urea accumulation, few methods can be applied to industrial food production due to potential safety concerns. In this study, adaptive laboratory evolution (ALE) followed by high-throughput screening was used to identify low urea-accumulating strains derived from the industrial Chinese rice wine yeast strain Saccharomyces cerevisiae XZ-11. Three evolved strains were obtained that had 47.9%, 16.6%, and 12.4% lower urea concentrations than the wild-type strain. Comparative genomics analysis revealed that genes involved in carbon and nitrogen metabolism evolved quickly. Transcription levels of genes involved in urea metabolism were dramatically upregulated after ALE. This work describes a novel and safe strategy to improve nitrogen utilization of industrial yeast strains involved in food fermentation. The identified genomic variations may also help direct rational genetic engineering of nitrogen metabolism processes to achieve other goals.

Regulation of Sensing, Transportation, and Catabolism of Nitrogen Sources in Saccharomyces cerevisiae

Nitrogen is one of the most important essential nutrient sources for biogenic activities. Regulation of nitrogen metabolism in microorganisms is complicated and elaborate. For this review, the yeast Saccharomyces cerevisiae was chosen to demonstrate the regulatory mechanism of nitrogen metabolism because of its relative clear genetic background. Current opinions on the regulation processes of nitrogen metabolism in S. cerevisiae, including nitrogen sensing, transport, and catabolism, are systematically reviewed. Two major upstream signaling pathways, the Ssy1-Ptr3-Ssy5 sensor system and the target of rapamycin pathway, which are responsible for sensing extracellular and intracellular nitrogen, respectively, are discussed. The ubiquitination of nitrogen transporters, which is the most general and efficient means for controlling nitrogen transport, is also summarized. The following metabolic step, nitrogen catabolism, is demonstrated at two levels: the transcriptional regulation process related to GATA transcriptional factors and the translational regulation process related to the general amino acid control pathway. The interplay between nitrogen regulation and carbon regulation is also discussed. As a model system, understanding the meticulous process by which nitrogen metabolism is regulated in S. cerevisiae not only could facilitate research on global regulation mechanisms and yeast metabolic engineering but also could provide important insights and inspiration for future studies of other common microorganisms and higher eukaryotic cells.

Complete genome sequence and analysis of the industrial Saccharomyces cerevisiae strain N85 used in Chinese rice wine production

Chinese rice wine is a popular traditional alcoholic beverage in China, while its brewing processes have rarely been explored. We herein report the first gapless, near-finished genome sequence of the yeast strain Saccharomyces cerevisiae N85 for Chinese rice wine production. Several assembly methods were used to integrate Pacific Bioscience (PacBio) and Illumina sequencing data to achieve high-quality genome sequencing of the strain. The genome encodes more than 6,000 predicted proteins, and 238 long non-coding RNAs, which are validated by RNA-sequencing data. Moreover, our annotation predicts 171 novel genes that are not present in the reference S288c genome. We also identified 65,902 single nucleotide polymorphisms and small indels, many of which are located within genic regions. Dozens of larger copy-number variations and translocations were detected, mainly enriched in the subtelomeres, suggesting these regions may be related to genomic evolution. This study will serve as a milestone in studying of Chinese rice wine and related beverages in China and in other countries. It will help to develop more scientific and modern fermentation processes of Chinese rice wine, and explore metabolism pathways of desired and harmful components in Chinese rice wine to improve its taste and nutritional value.

CRISPR/Cas9 System as a Valuable Genome Editing Tool for Wine Yeasts with Application to Decrease Urea Production

An extensive repertoire of molecular tools is available for genetic analysis in laboratory strains of S. cerevisiae. Although this has widely contributed to the interpretation of gene functionality within haploid laboratory isolates, the genetics of metabolism in commercially-relevant polyploid yeast strains is still poorly understood. Genetic engineering in industrial yeasts is undergoing major changes due to Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and CRISPR-associated protein (Cas) engineering approaches. Here we apply the CRISPR/Cas9 system to two commercial “starter” strains of S. cerevisiae (EC1118, AWRI796), eliminating the CAN1 arginine permease pathway to generate strains with reduced urea production (18.5 and 35.5% for EC1118 and AWRI796, respectively). In a wine-model environment based on two grape musts obtained from Chardonnay and Cabernet Sauvignon cultivars, both S. cerevisiae starter strains and CAN1 mutants completed the must fermentation in 8–12 days. However, recombinant strains carrying the can1 mutation failed to produce urea, suggesting that the genetic modification successfully impaired the arginine metabolism. In conclusion, the reduction of urea production in a wine-model environment confirms that the CRISPR/Cas9 system has been successfully established in S. cerevisiae wine yeasts.

A Bacillus paralicheniformis Iron-Containing Urease Reduces Urea Concentrations in Rice Wine

Urease, a nickel-containing metalloenzyme, was the first enzyme to be crystallized and has a prominent position in the history of biochemistry. In the present study, we identified a nickel urease gene cluster, ureABCEFGDH, in Bacillus paralicheniformis ATCC 9945a and characterized it in Escherichia coli Enzymatic assays demonstrate that this oxygen-stable urease is also an iron-containing acid urease. Heterologous expression assays of UreH suggest that this accessory protein is involved in the transmembrane transportation of nickel and iron ions. Moreover, this iron-containing acid urease has a potential application in the degradation of urea in rice wine. The present study not only enhances our understanding of the mechanism of activation of urease but also provides insight into the evolution of metalloenzymes.IMPORTANCE An iron-containing, oxygen-stable acid urease from B. paralicheniformis ATCC 9945a with good enzymatic properties was characterized. This acid urease shows activities toward both urea and ethyl carbamate. After digestion with 6 U/ml urease, approximately 92% of the urea in rice wine was removed, suggesting that this urease has great potential in the food industry.

Mutant Potential Ubiquitination Sites in Dur3p Enhance the Urea and Ethyl Carbamate Reduction in a Model Rice Wine System

Ubiquitination can significantly affect the endocytosis and degradation of plasma membrane proteins. Here, the ubiquitination of a Saccharomyces cerevisiae urea plasma membrane transporter (Dur3p) was altered. Two potential ubiquitination sites, lysine residues K556 and K571, of Dur3p were predicted and replaced by arginine, and the effects of these mutations on urea utilization and formation under different nitrogen conditions were investigated. Compared with Dur3p, the Dur3p_K556R mutant showed a 20.1% decrease in ubiquitination level in yeast nitrogen base medium containing urea and glutamine. It also exhibited a >75.8% decrease in urea formation in yeast extract-peptone-dextrose medium and 41.3 and 55.4% decreases in urea and ethyl carbamate formation (a known carcinogen), respectively, in a model rice wine system. The results presented here show that the mutation of Dur3p ubiquitination sites could significantly affect urea utilization and formation. Modifying the ubiquitination of specific transporters might have promising applications in rationally engineering S. cerevisiae strains to efficiently use specific nitrogen sources.

Genome-wide mapping of nucleosome positions in Saccharomyces cerevisiae in response to different nitrogen conditions

Well-organized chromatin is involved in a number of various transcriptional regulation and gene expression. We used genome-wide mapping of nucleosomes in response to different nitrogen conditions to determine both nucleosome profiles and gene expression events in Saccharomyces cerevisiae. Nitrogen conditions influence general nucleosome profiles and the expression of nitrogen catabolite repression (NCR) sensitive genes. The nucleosome occupancy of TATA-containing genes was higher compared to TATA-less genes. TATA-less genes in high or low nucleosome occupancy, showed a significant change in gene coding regions when shifting cells from glutamine to proline as the sole nitrogen resource. Furthermore, a correlation between the expression of nucleosome occupancy induced NCR sensitive genes or TATA containing genes in NCR sensitive genes, and nucleosome prediction were found when cells were cultured in proline or shifting from glutamine to proline as the sole nitrogen source compared to glutamine. These results also showed that variation of nucleosome occupancy accompany with chromatin-dependent transcription factor could influence the expression of a series of genes involved in the specific regulation of nitrogen utilization.

CAR1 deletion by CRISPR/Cas9 reduces formation of ethyl carbamate from ethanol fermentation by Saccharomyces cerevisiae

Enormous advances in genome editing technology have been achieved in recent decades. Among newly born genome editing technologies, CRISPR/Cas9 is considered revolutionary because it is easy to use and highly precise for editing genes in target organisms. CRISPR/Cas9 technology has also been applied for removing unfavorable target genes. In this study, we used CRISPR/Cas9 technology to reduce ethyl carbamate (EC), a potential carcinogen, which was formed during the ethanol fermentation process by yeast. Because the yeast CAR1 gene encoding arginase is the key gene to form ethyl carbamate, we inactivated the yeast CAR1 gene by the complete deletion of the gene or the introduction of a nonsense mutation in the CAR1 locus using CRISPR/Cas9 technology. The engineered yeast strain showed a 98 % decrease in specific activity of arginase while displaying a comparable ethanol fermentation performance. In addition, the CAR1-inactivated mutants showed reduced formation of EC and urea, as compared to the parental yeast strain. Importantly, CRISPR/Cas9 technology enabled generation of a CAR1-inactivated yeast strains without leaving remnants of heterologous genes from a vector, suggesting that the engineered yeast by CRISPR/Cas9 technology might sidestep GMO regulation.

Formation of ethyl carbamate in Goji wines: Effect of Goji fruit composition

Ethyl carbamate (EC) is a multisite carcinogen widely occurring in alcoholic beverages. In this investigation, solid-phase extraction combined with gas chromatography mass spectrometry was employed to determine EC contents during the fermentation and storage processes, and the effects of Goji varieties on its formation were also examined. The results indicated that natural and potential EC contents were significantly affected by the varied composition of Goji fruits. The analysis of chemical properties showed differences in hundred-grain weight, water contents, amino acids, and nitrogen-to-carbon ratio for Goji berries. Citrulline was completely degraded although it is routinely considered as a non-preferred nitrogen for yeasts. Due to compositional differences, Goji wines accumulated distinct urea levels that positively correlated with the potential EC contents. Furthermore, the temperature in both the production processes highly influenced EC formation. These results contribute to a more comprehensive understanding of EC formation, and in turn, controlling EC in the Goji wine matrix.

The modification of Gat1p in nitrogen catabolite repression to enhance non-preferred nitrogen utilization in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, when preferred nitrogen sources are present, the metabolism of non-preferred nitrogen is repressed. Previous work showed that this metabolic regulation is primarily controlled by nitrogen catabolite repression (NCR) related regulators. Among these regulators, two positive regulators (Gln3p and Gat1p) could be phosphorylated and sequestered in the cytoplasm leading to the transcription of non-preferred nitrogen metabolic genes being repressed. The nuclear localization signals (NLSs) and nuclear localization regulatory signals (NLRSs) in Gln3p and Gat1p play essential roles in the regulation of their localization in cells. However, compared with Gln3p, the information of NLS and NLRS for Gat1p remains unknown. In this study, residues 348-375 and 366-510 were identified as the NLS and NLRS of Gat1p firstly. In addition, the modifications of Gat1p (mutations on the NLS and truncation on the NLRS) were attempted to enhance the transcription of non-preferred nitrogen metabolic genes. Quantitative real-time PCR showed that the transcriptional levels of 15 non-preferred nitrogen metabolic genes increased. Furthermore, during the shaking-flask culture tests, the utilization of urea, proline and allantoine was significantly increased. Based on these results, the genetic engineering on Gat1p has a great potential in enhancing non-preferred nitrogen metabolism in S. cerevisiae.

Effects of three permeases on arginine utilization in Saccharomyces cerevisiae

Arginine plays an important role in cellular function and metabolism. Arginine uptake mainly occurs through three amino acid permeases, Alp1p, Gap1p and Can1p, which act as both transporters and receptors for amino acid utilization. In this study, seven mutants were constructed with different combinations of permease deficiencies that inhibit arginine utilization. Their effects on arginine metabolism were measured. The three amino acid permeases were also individually overexpressed in wild-type (WT), Δalp1Δgap1Δcan1 and Δnpr1 strains. The growth and arginine utilization of Δcan1, Δgap1Δcan1 and Δalp1Δgap1Δcan1 mutants were suppressed in YNB medium when arginine was the sole nitrogen source. Meanwhile, overexpression of Alp1p and Can1p enhanced growth and arginine utilization in WT, Δalp1Δgap1Δcan1 and Δnpr1. Besides, overexpression of Can1p caused a 26.7% increase in OD600 and 29.3% increase in arginine utilization compared to that of Alp1p in Δalp1Δgap1Δcan1. Transcription analysis showed that the effects of three amino acid permeases on the arginine utilization and the regulation of related genes, were tightly related to their individual characteristics. However, their overall effects were different for different combinations of mutants. The results presented here suggest some possible synergistic effects of different amino acid permeases on regulation of amino acid utilization and metabolism.

Mitigating health risks associated with alcoholic beverages through metabolic engineering

Epidemiological studies have established a positive relationship between the occurrence of cancer and consumption of alcoholic beverages. Metabolic engineering of brewing yeast to reduce potential carcinogenic compounds in alcoholic beverage is technically feasible as well as economically promising. This review presents the mechanisms of formation of potentially carcinogenic components in alcoholic beverages, such as formaldehyde, acetaldehyde, ethyl carbamate, acrylamide, and heavy metals, and introduces effective genetic perturbations to minimize the concentrations of these harmful components. As precise and effective genome editing tools for polyploid yeast are now available, we envision that yeast metabolic engineering might open up new research directions for improving brewing yeast in order to ensure product safety as well as to increase overall quality of alcoholic beverages.

An improved and validated sample cleanup method for analysis of ethyl carbamate in Chinese liquor

Ethyl carbamate (EC) is a potential human carcinogen widely existing in fermented foods and alcoholic beverages. The solid-phase extraction (SPE) coupled to gas chromatography mass spectrometry is a widely-used method to determine EC levels, but the accuracy varies with sample matrix and the effects of operation parameters are rarely examined. In this study, the influence factors involved in EC determination were investigated using Chinese liquor as sample matrix, and the improved method was further applied. Three types of SPE columns, including diatomite, Florisil, and primary-secondary amine, were compared in extraction efficiency, and the diatomite column exhibited the highest extraction efficiency. The optimal volumes of elution solvents with diatomite column were 15 mL for 3-mL samples solution loaded. In addition, the alcoholic strength for EC determination should be diluted below 20% (v/v) to avoid the enhancement of matrix-induced chromatographic response. Moreover, the pH neutralization could help improve EC recovery and peak resolution, reducing interfering effects. Based on these results, the improved method showed that the limit of detection, the limit of quantification, and average recoveries were 1.10 μg/L, 3.65 μg/L, and 93.06%, respectively. To further elucidate the underlying factors related to EC accumulation, partial least square regression analysis was conducted, and the results suggested that EC levels had the closest relationship with alcoholic strength among the remaining precursors.

High-level expression and characterization of recombinant acid urease for enzymatic degradation of urea in rice wine

Ethylcarbamate, a carcinogenic compound, is formed from urea and ethanol in rice wine, and enzymatic elimination of urea is always attractive. In the present work, we amplified the acid urease gene cluster ureABCEFGD from Lactobacillus reuteri CICC6124 and constructed robust Lactococcus lactis cell factories for the production of acid urease. The titer of the recombinant acid urease was increased from 1,550 to 11,560 U/L by optimization of the cultivation process. Meanwhile, the enzyme showed satisfied properties toward urea elimination in the rice wine model system. By incubating the enzyme (50 U/L) at 20 °C for 60 h, about 95.8% of urea in rice wine was removed. Interestingly, this acid urease also exhibited activity toward ethylcarbamate. The results demonstrated that this recombinant acid urease has great potential in the elimination of urea in rice wine.