Abstract
We aimed to produce intrinsically L-[1-13C]phenylalanine labeled milk and beef for subsequent use in human nutrition research. The collection of the various organ tissues after slaughter allowed for us to gain insight into the dynamics of tissue protein turnover in vivo in a lactating dairy cow. One lactating dairy cow received a constant infusion of L-[1-13C]phenylalanine (450 µmol/min) for 96 h. Plasma and milk were collected prior to, during, and after the stable isotope infusion. Twenty-four hours after cessation of the infusion the cow was slaughtered. The meat and samples of the various organ tissues (liver, heart, lung, udder, kidney, rumen, small intestine, and colon) were collected and stored. Approximately 210 kg of intrinsically labeled beef (bone and fat free) with an average L-[1-13C]phenylalanine enrichment of 1.8±0.1 mole percent excess (MPE) was obtained. The various organ tissues differed substantially in L-[1-13C]phenylalanine enrichments in the tissue protein bound pool, the highest enrichment levels were achieved in the kidney (11.7 MPE) and the lowest enrichment levels in the skeletal muscle tissue protein of the cow (between 1.5–2.4 MPE). The estimated protein synthesis rates of the various organ tissues should be regarded as underestimates, particularly for the organs with the higher turnover rates and high secretory activity, due to the lengthened (96 h) measurement period necessary for the production of the intrinsically labeled beef. Our data demonstrates that there are relatively small differences in L-[1-13C]phenylalanine enrichments between the various meat cuts, but substantial higher enrichment values are observed in the various organ tissues. We conclude that protein turnover rates of various organs are much higher when compared to skeletal muscle protein turnover rates in large lactating ruminants.
Highlights
Protein intake stimulates muscle protein synthesis rates [1]
The various organ tissues differed in their observed L-[1-13C]phenylalanine enrichments which emphasizes the large variance in tissue protein turnover rates
The highest enrichments were achieved in the kidney (11.7 mole percent excess (MPE)), udder (10.3 MPE), and the liver (9.4 MPE), whereas L[1-13C]phenylalanine enrichments were lower in cardiac (4.7 MPE) and skeletal muscle tissue of the cow
Summary
The digestion and absorption kinetics of the ingested dietary protein, and the subsequent release of amino acids in the circulation, modulates the amplitude of the stimulation of postprandial muscle protein synthesis rates [2,3]. Innovative work has demonstrated the feasibility of producing intrinsically labeled milk for the use in human nutrition research [5,6,7]. Our past efforts established that it is feasible to produce intrinsically labeled meat for the assessment of protein digestion and amino acid absorption kinetics in vivo in humans [7]. The labeled meat provides researchers with another protein-rich food source to develop novel nutritional strategies aimed at maximizing postprandial skeletal muscle protein accretion
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