title: Influence of peptides and amino acids on fermentation rate and de novo synthesis of amino acids by mixed micro-organisms from the sheep rumen.
abstract: The influence of different N sources on fermentation rate and de novo amino acid synthesis by rumen micro-organisms was investigated in vitro using rumen fluid taken from four sheep receiving a mixed diet comprising (g/kg DM): grass hay 500, barley 299.5, molasses 100, fish meal 91, minerals and vitamins 9.5. Pancreatic casein hydrolysate (P; comprising mainly peptides with some free amino acids; 10 g/l), free amino acids (AA; casein acid hydrolysate + added cysteine and tryptophan; 10 g/l), or a mixture of L-proline, glycine, L-valine and L-threonine (M; 0.83 g/l each) were added to diluted (1:3, v/v), strained rumen fluid along with 15NH4Cl (A; 1.33 g/l) and 6.7 g/l of a mixture of starch, cellobiose and xylose (1:1:1, by weight). P and AA, but not M, stimulated net gas production after 4 and 8 h incubation (P < 0.05) in comparison with A alone. P increased microbial-protein synthesis (P < 0.05) compared with the other treatments. All of the microbial-N formed after 10 h was synthesized de novo from 15NH3 in treatment A, and the addition of pre-formed amino acids decreased the proportion to 0.37, 0.55, and 0.86 for P, AA, and M respectively. De novo synthesis of amino acids (0.29, 0.42 and 0.69 respectively) was lower than cell-N. Enrichment of alanine, glutamate and aspartate was slightly higher than that of other amino acids, while enrichment in proline was much lower, such that 0.83-0.95 of all proline incorporated into particulate matter was derived from pre-formed proline. Glycine, methionine, lysine, valine and threonine tended to be less enriched than other amino acids. The form in which the amino acids were supplied, as P or AA, had little influence on the pattern of de novo synthesis. When the concentration of peptides was decreased, the proportion of microbial-N formed from NH3 increased, so that at an initial concentration of 1 g peptides/l, similar to the highest reported ruminal peptide concentrations, 0.68 of cell-N was formed from NH3. Decreasing the NH3 concentration at 1.0 g peptides/l caused proportionate decreases in the fraction of cell-N derived from NH3, from 0.81 at 0.53 g NH3-N/l to 0.40 at 0.19 g NH3-N/l. It was concluded that different individual amino acids are synthesized de novo to different extents by mixed rumen micro-organisms when pre-formed amino acids are present, and that the source of N used for synthesis of cell-N and amino acids depends on the respective concentrations of the different N sources available; however, supplementing only with amino acids whose synthesis is lowest when pre-formed amino acids are present does not stimulate fermentation or microbial growth.
authors.affiliation: Rowett Research Institute, Bucksburn, Aberdeen, UK.
authors.forename: C
authors.initials: C
authors.lastname: Atasoglu
authors.forename: C
authors.initials: C
authors.lastname: Valdés
authors.forename: C J
authors.initials: CJ
authors.lastname: Newbold
authors.forename: R J
authors.initials: RJ
authors.lastname: Wallace
mesh_terms: D000596:Amino Acids / Q000096:biosynthesis / Q000494:pharmacology*; D000641:Ammonia / Q000378:metabolism; D000821:Animal Feed; D000818:Animals; D001419:Bacteria / Q000378:metabolism*; D001426:Bacterial Proteins / Q000096:biosynthesis; D019587:Dietary Supplements; D005285:Fermentation; D009584:Nitrogen / Q000378:metabolism; D010455:Peptides / Q000494:pharmacology*; D012417:Rumen / Q000382:microbiology*; D012756:Sheep
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; D010455:Peptides; D000641:Ammonia; D009584:Nitrogen