title: Measuring protein breakdown rate in individual proteins in vivo.
abstract: OBJECTIVE
  To outline different approaches of how protein breakdown can be quantified and to present a new approach to determine the fractional breakdown rate of individual slow turnover proteins in vivo.
  
  
  RESULTS
  None of the available methods for determining protein breakdown can be used to determine the breakdown rate of specific proteins and, therefore, do not keep up to the preceding methodological demands in physiological research. A newly developed approach to determine the fractional breakdown rate of single proteins seems promising. Its conceptual advantage is that the proteins of interest are the site of measurement. Hence, the application initially demands the proteins to be labeled with stable isotopically labeled amino acids. Subsequently, the loss of label from the proteins will be dependent on the protein breakdown rate when no labeled amino acids are reincorporated into the protein, the protein mass is steady, and when proteins contained in the measured fraction are stochastically selected for degradation.
  
  
  CONCLUSIONS
  Although the synthesis rate of specific proteins can be accurately determined, methodological improvements are required to elucidate the physiological role of protein degradation. The novel approach is promising but future studies are needed to address its wider applicability.
authors.affiliation: Institute of Sports Medicine and Department of Orthopedic Surgery, Bispebjerg Hospital and Center of Healthy Aging, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark. L.Holm.isotope@gmail.com
authors.forename: Lars
authors.initials: L
authors.lastname: Holm
authors.forename: Michael
authors.initials: M
authors.lastname: Kjaer
mesh_terms: D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D006801:Humans; D007553:Isotope Labeling; D011506:Proteins / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D011506:Proteins