title: Dietary Methyl Donors Contribute to Whole-Body Protein Turnover and Protein Synthesis in Skeletal Muscle and the Jejunum in Neonatal Piglets.
abstract: BACKGROUND
  The neonatal methionine requirement must consider not only the high demand for rapid tissue protein expansion but also the demands as the precursor for a suite of critical transmethylation reactions. However, methionine metabolism is inherently complex because upon transferring its methyl group during transmethylation, methionine can be reformed by the dietary methyl donors choline (via betaine) and folate.
  
  
  OBJECTIVE
  We sought to determine whether dietary methyl donors contribute to methionine availability for protein synthesis in neonatal piglets.
  
  
  METHODS
  Yucatan miniature piglets aged 4-8 d were fed a diet that provided 38 μg folate/(kg·d), 60 mg choline/(kg·d), and 238 mg betaine/(kg·d) [methyl-sufficient (MS); n = 8] or a diet devoid of these methyl precursors [methyl-deficient (MD); n = 8]. After 5 d, dietary methionine was reduced from 0.30 to 0.20 g/(kg·d) in both groups. On day 6, piglets received a constant [1-13C]phenylalanine infusion to measure whole-body protein kinetics, and on day 8 they received a constant [3H-methyl]methionine infusion to measure tissue-specific protein synthesis in skeletal muscle, the liver, and the jejunum.
  
  
  RESULTS
  Whole-body phenylalanine flux, protein synthesis, and protein breakdown were 13%, 12%, and 22% lower, respectively, in the MD group than in the MS group (P < 0.05). Reduced whole-body protein synthesis in the MD piglets was attributed to 50% lower protein synthesis in skeletal muscle and the jejunum than in the MS piglets (P < 0.05). Furthermore, methionine availability in skeletal muscle was halved in piglets fed the MD diet (P < 0.05), and the specific radioactivity of methionine was doubled in the jejunum of MD piglets (P < 0.05), suggesting lower intestinal remethylation. Liver protein synthesis did not significantly differ between the groups, but secreted proteins were not measured.
  
  
  CONCLUSIONS
  Dietary methyl donors can affect whole-body and tissue-specific protein synthesis in neonatal piglets and should be considered when determining the methionine requirement.
authors: Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador, Canada; and.
authors: Jason L
authors: JL
authors: Robinson
authors: Faculty of Life Sciences and Medicine, Diabetes and Nutritional Sciences Division, King's College London, London, United Kingdom.
authors: Scott V
authors: SV
authors: Harding
authors: Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador, Canada; and.
authors: Janet A
authors: JA
authors: Brunton
authors: Department of Biochemistry, Memorial University of Newfoundland, St. John's, Newfoundland and Labrador, Canada; and rbertolo@mun.ca.
authors: Robert F
authors: RF
authors: Bertolo
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D001622:Betaine / Q000008:administration & dosage; D002794:Choline / Q000008:administration & dosage; D004032:Diet; D004305:Dose-Response Relationship, Drug; D005260:Female; D005492:Folic Acid / Q000008:administration & dosage; D007583:Jejunum / Q000378:metabolism*; D008297:Male; D008715:Methionine / Q000008:administration & dosage / Q000031:analogs & derivatives*; D018482:Muscle, Skeletal / Q000378:metabolism*; D010649:Phenylalanine / Q000008:administration & dosage; D014176:Protein Biosynthesis; D013552:Swine
chemical_list: D001622:Betaine; D010649:Phenylalanine; D005492:Folic Acid; D008715:Methionine; C038344:methionine methyl ester; D002794:Choline
keywords: 1-carbon metabolism; isotope kinetics; methionine requirement; protein metabolism; remethylation