title: Plasma protein synthesis after spaceflight.
abstract: BACKGROUND
  Astronauts land in a protein-depleted state. An anabolic phase takes place during the postflight period as muscle regains the lost protein. Yet where dietary intake has been measured after spaceflight, there does not appear to be any significant increase in dietary protein intake relative to preflight to provide additional amino acids to support muscles as they regain protein. We hypothesized that protein synthesis in other tissues is sub-optimal after spaceflight because of substrate competition for amino acids occurring between the muscles needing protein and other tissues.
  
  
  METHODS
  We measured selected plasma protein synthesis rates before and after spaceflight on the Shuttle using the 15N glycine-hippuric acid method. The fractional protein synthesis rates (FSR) of four plasma proteins, fibrinogen, complement C-3, ceruloplasmin, and haptoglobin, were measured before and after a 16-d flight on the Space Shuttle. Data was obtained for four subjects. Preflight measurements of plasma protein synthesis rates were made 45 and 7 d before launch. Postflight measurements were done on the day of landing and 6 and 14 d later.
  
  
  RESULTS
  Compared with preflight, plasma protein synthesis rates were reduced 6 d after landing.
  
  
  CONCLUSIONS
  Plasma protein synthesis rates are depressed after spaceflight. The observations are consistent with amino acids being the limiting factor due to substrate competition between the muscles needing protein and other tissues.
authors.affiliation: Department of Surgery, University of Medicine and Dentistry of New Jersey - SOM, Stratford, NJ 08084, USA. tpstein@umdnj.edu
authors.forename: T Peter
authors.initials: TP
authors.lastname: Stein
authors.forename: Margaret D
authors.initials: MD
authors.lastname: Schluter
mesh_terms: D018480:Astronauts; D001798:Blood Proteins / Q000096:biosynthesis*; D002570:Ceruloplasmin / Q000032:analysis; D003176:Complement C3 / Q000032:analysis; D002149:Energy Intake; D005340:Fibrinogen / Q000032:analysis; D006242:Haptoglobins / Q000032:analysis; D006801:Humans; D047468:Immunoprecipitation; D013026:Space Flight; D013312:Stress, Physiological / Q000097:blood
chemical_list: D001798:Blood Proteins; D003176:Complement C3; D006242:Haptoglobins; D005340:Fibrinogen; D002570:Ceruloplasmin