Interestingly genome wide association studies
Interestingly, genome-wide association studies have linked the chromosomal region harboring RDHE2 (SDR16C5) and seven other genes to stature and growth in cattle, humans, and pigs , , , , , , , , and beak deformity in chickens . The most recent study specifically identified RDHE2 as the important candidate gene in pig growth trait by an integrative genomic approach . The results of our study suggest that this phenotype may be related to its activity as a retinol dehydrogenase.
This work was supported by the National Institute on Alcohol Abuse and Alcoholism Grant AA12153 to N.Y.K. and by NIH T32 fellowship GM008111 to M.K.A.
Introduction Xylitol is a naturally occurring pentahydroxy sugar alcohol, which has a wide variety of applications in food and pharmaceuticals industries (Dasgupta et al., 2017, de Albuquerque et al., 2015). Since xylitol provides rapid sweetness, flavor, quick cooling effect and as a low-energy sweetener, it has been popularly used in the formulation of various food and confectionery products, such as sugar-free chewing gum and bakery products. Additionally, xylitol can be used as a sweetener in many pharmaceutical preparations as a substitute of sucrose (Jia et al., 2016). Xylitol-sweetened medications are used to prevent dental caries as it causes very little increase in glucose level and insulin in blood (Cardoso et al., 2016). Currently lignocellulosic DNQX disodium salt based biorefineries produced a variety of value added products, where xylitol has been placed as one of the 12 important value-added products by the US Department of Energy (Ghaffar et al., 2017, Venkateswar Rao et al., 2016). Due to the growing demand of xylitol in food, pharmaceuticals and odontological industries, consumption of xylitol has been increased dramatically in recent years, which was roughly 160,000 metric tons in 2013 and is expected to be 242,000 metric tons by 2020 (Rao et al., 2016). Likewise, the market value of xylitol is also rising significantly day by day as estimated for US$ 670 million in 2013 to US$ 1 billion by 2020 with the selling price of US$ 4.5–5.5/kg for bulk in industries and US$ 20/kg as the final product in the supermarkets (Rao et al., 2016, Ravella et al., 2012). Although there is a growing interest in the biological production of xylitol from renewable materials (Albuquerque et al., 2014, Dhar et al., 2016), at present, most of the xylitol is produced on commercial scale by traditional chemical approach that include hydrogenation of pure d-xylose derived from the biomass (Ur-Rehman et al., 2015). Even though d-xylose can be obtained from cheap lignocellulosic (de Albuquerque et al., 2015, Zabed et al., 2016), the traditional chemical route of xylitol production is very costly due to some drawbacks and economic issues. Firstly, it requires high cost Raney-nickel catalyst together with high pressure (10–15 atm) and elevated temperature of up to 130 °C (Cheng et al., 2014). Moreover, chemical synthesis of xylitol employs not only the expensiveness process, but also requires high costs and high amounts of water for purification of xylose from hemicellulose hydrolysates, in addition to the requirement of costly refining process for downstream xylitol recovery (Albuquerque et al., 2014, Pal et al., 2016). What is more, chemical synthesis of xylitol is not environmentally friendly and is energy consuming (Dasgupta et al., 2017, Li et al., 2015). Alternatively, microbial biotransformation of xylose into xylitol has been investigated using several yeast strains such as Debaryomyces hansenii, Candida tropicalis and Candida parapsilosis (Li et al., 2016). This bioconversion route is further found to be challenging because of the requirement of hydrolysis of hemicellulose and purification of xylose from the hydrolysate as it contains microbial inhibitors (Li et al., 2016). Compared to xylose, glucose could be a good candidate for xylitol production in the perspectives of the technology and economy (Mayer et al., 2002). However, natural microorganisms cannot convert glucose to xylitol directly due to the lack of comprehensive metabolic pathway and hence, it is necessary to employ a two-step biotransformation process, in which glucose is first converted into d-arabitol by yeasts that undergoes subsequent biotransformation into xylitol by microorganisms (Li et al., 2016, Qi et al., 2014). This two-step biotransformation system is therefore technologically complex and time consuming. As a result, although d-arabitol is a relatively costly substrate compared to glucose, it has some promising advantages and can be used for xylitol production by the most efficient xylitol producing strains, such as Gluconobacter sp., which do not have the metabolic system to convert glucose into d-arabitol, in addition to the fact that utilization of d-arabitol as substrate reduces process step and risk of contamination (Zhang et al., 2013, Qi et al., 2015). Additionally, d-arabitol can be produced from various renewable sources such as glucose, glycerol and xylose by simple microbial biotransformation (Kordowska-Wiater, 2015, Yoshikawa et al., 2014, Zhu et al., 2010).