title: Kinetics of microbial methionine metabolism in continuous cultures administered different methionine sources.
abstract: The Met precursor 2-hydroxy-4-(methylthio) butanoic acid (HMB) is expected to be more extensively degraded in the rumen than its isopropyl ester (HMBi). A control and 3 isomolar treatments-0.097% dl-methionine, 0.11% HMBi (HMBi), and 0.055% HMBi plus 0.048% Met (Met + HMBi)-were dosed every 8h simultaneously with 3-times-daily feeding into continuous cultures. Starting on d 9, for 6 consecutive doses, both [1-(13)C]-l-Met and [methyl-(2)H3]-l-Met replaced part of the unlabeled dl-Met, [(13)C5]-dl-HMBi replaced a portion of the unlabeled dl-HMBi, and [1-(13)C]-l-Met plus [(13)C5]-dl-HMBi replaced a portion of the respective unlabeled doses for the Met + HMBi treatment. After the sixth dose (d 11), unlabeled Met or HMBi provided 100% of the doses to follow elimination kinetics of the labels in HMBi, free Met, and bacterial Met compartments. The free [1-(13)C]-l-Met recycled more and was recovered in bacterial Met to a lesser extent than was the free [methyl-(2)H3]-l-Met recycling and that was recovered in bacterial Met. Increasing HMBi inclusion (0, 50, and 100% substitution of the exogenously dosed Met on a molar equivalent basis) tended to increase HMBi escape from 54.7 to 71.3% for the 50 and 100% HMBi treatments, respectively. Despite HMBi substituting for and decreasing the dosage of Met, increasing HMBi increased accumulation of free Met in fermenter fluid. The HMBi (after de-esterification of the isopropyl group) presumably produces Met through the intermediate α-ketomethylthyiobutyrate with an aminotransferase that also has high affinity for branched-chain AA. We provide evidence that the HMBi-derived Met is likely released from bacterial cells and accumulates rather than being degraded, potentially as a result of lagging d-stereoisomer metabolism. More research is needed to evaluate racemization and metabolism of stereoisomers of HMBi, Met, and other AA in ruminal microbes.
authors.affiliation: Department of Animal Sciences, The Ohio State University, Columbus 43210. Electronic address: firkins.1@osu.edu.
authors.forename: J L
authors.initials: JL
authors.lastname: Firkins
authors.affiliation: Department of Animal Sciences, The Ohio State University, Columbus 43210.
authors.forename: C M
authors.initials: CM
authors.lastname: Fowler
authors.affiliation: Adisseo Inc., 03600 Commentry, France.
authors.forename: E
authors.initials: E
authors.lastname: Devillard
authors.affiliation: Department of Animal and Avian Sciences, University of Maryland, College Park 20742.
authors.forename: B J
authors.initials: BJ
authors.lastname: Bequette
mesh_terms: D000818:Animals; D001419:Bacteria / Q000378:metabolism*; D001431:Bacteriological Techniques; D004951:Esterification; D007700:Kinetics; D008715:Methionine / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism*; D012417:Rumen / Q000378:metabolism* / Q000382:microbiology*; D013237:Stereoisomerism
chemical_list: C008391:alpha-hydroxy-gamma-methylmercaptobutyric acid; D008715:Methionine
keywords: hydroxymethylthiobutanoic acid; methionine; microbial methionine metabolism