Biofuel co-products as livestock feed - Opportunities and challenges

104Biofuel co-products as livestock feed – Opportunities and challengesFIGURE 1Possible metabolic pathways for H 2 S productionHypotaurineCSDCysteine sulfinateL-Serine + H 2SCO 2 Cystathionine + H 2SLanthionine + H 2SO 2 HomocysteineH 2OCysteineL-MethionineCDO CBS CBS CBSCysSR + HProtein2SCSECSER-SHCBSCSEHomocysteineCystathionine CysteineCystine ThiocysteineSerineHomocysteineα-KetobutyrateCSEPyruvate + NH+ NH 33H 2SKeto AcidCysteineHomolanthionine + H 2SCSE CLYCATR-SH2-SO AminoH 2SH 2O3AcidPyruvate + NH 3+ H 2SL-Cysteate + H 2SCysteine ThioetherPyruvate+ H 2S3-MST3-MercaptopyruvateSO 32-PyruvateGSSG + SO 32-+ H 2SThiosulfate cycle SSO 32-Abbreviations: CA = carbonic anhydrase; CBS = cystathionine β-synthase; CDO = cysteine dioxygenase; CSD = cysteinesulphinate decarboxylase; CSE =cystathionine γ-ligase; MST = 3-mercaptopyruvate sulphurtransferase.Source: Adapted from Olson, 2011.(CSE) and 3-mercaptopyruvate sulphurtransferase (MST).Cystathionine β-synthase and CSE catalyze severaltrans sulphuration reactions of a multitude of substratecombinations, whereas MST deaminates cysteine to formmercaptopyruvate, which is subsequently convertedto pyruvate and H 2 S. The prevalence of CBS, CSE andMST in the different tissues of the animal body varies.For example, CBS was shown to be the predominantenzymatic pathway for H 2 S in brain and CSE wasthe major pathway in the vasculature (Olson, 2011).Hydrogen sulphide also is produced in the vascularsmooth muscle by the pathway involving MST. Generallyconsidered the major catabolic pathway for cysteine,cysteine dioxygenase (CDO) catalyzes the additionof O 2 to cysteine to form cysteinesulphinate that issubsequently decarboxylated to hypotaurine (Stipanukand Ueki, 2010). The action of CDO is considered themajor physiological regulator of intracellular cysteineavailability. By oxidizing excess cysteine, the CDO maybe in important physiological regulator of endogenousH 2 S production. Future research is needed to associatethe pathway for synthesis of H 2 S in the myriad of tissuesof an animal for association of this signal molecule tospecific physiological functions.Sulphate reduction to H 2 S by ruminal bacteriaAlthough sulphur amino acids can be catabolized by mammaliancells into H 2 S, it is well established that reductionof inorganic sulphate to H 2 S does not occur in mammaliancells. Sulphate reduction to H 2 S does occur in sulphatereducingbacteria, which are present in both the ruminantand non-ruminant digestive tracts (NRC, 2005). Sulphurreducingbacteria in the rumen utilize anaerobic respirationpathways for bio-energetic processes. Bacteria in therumen can metabolize S as elemental, inorganic or organicS. Two metabolic pathways have been proposed for dietaryS in the rumen: the assimilatory and dissimilatory pathways(Cummings et al., 1995). The assimilatory pathway is thereduction of sulphate to sulphide and its incorporationinto S-containing compounds (e.g. cysteine and methionine)destined for use in microbial proteins. Assimilatorybacteria include bacteria from the Bacteroides, Butyvibrioand Lachnospira genera (Cummings et al., 1995). Thedissimilatory pathway is used by some rumen microbes toderive energy from the reduction of sulphate to H 2 S; H 2 Sthen is released into the rumen gas cap. Both assimilatoryand dissimilatory sulphate reductions are carried out byanaerobic ruminal bacteria. However, reduction to H 2 S predominatesin the rumen (Cummings et al., 1995). Although