Improving nitrogen utilization in the ruminant and consequently minimizing the environmental impacts of ruminant production are dependent on a clear understanding of postabsorptive amino acid metabolism. The objective of this study was to develop an isotope dilution model to describe the partitioning of phenylalanine (PHE) and tyrosine (TYR) in the bovine liver. The model builds upon the existing eight-pool model to describe protein turnover in the mammary gland of the lactating dairy cow (Crompton et al. 2014). The present model was composed of four intracellular and six extracellular pools and various flows connecting these pools and the external system (Fig. 4). Principles of mass conservation were applied to generate the fundamental equations describing the behavior of the system in the steady state. The model was applied to data sets from five multicatheterized dairy cows during a constant infusion of [1-13C] PHE and [2,3,5,6-2H] TYR tracers. Consistent with in vivo findings, net flux of PHE was negative, suggesting that the liver is a major site of PHE catabolism. A unique feature of the present model is the description of intracellular PHE and TYR partitioning. Constitutive hepatic protein degradation was the largest contributor to the free PHE and TYR pools, followed by portal vein and hepatic artery delivery (PHE 55%, 40%, 5%; TYR 58%, 37%, 5%). Flow of PHE and TYR into constitutive protein synthesis comprised 97% of total synthetic flow. The ratio of constitutive protein synthesis/degradation was 1.4 for PHE and 0.9 for TYR. The model predicted negligible PHE to TYR conversion rates (<2%) suggesting that PHE was not oxidized to any great extent. This finding is an apparent limitation of the current modelling exercise. Overall, the model was effective in providing information about the fates of PHE and TYR in the liver and could be used as part of a more complex system describing amino acid metabolism in the whole animal.
|Journal||Canadian Journal of Animal Science|
|Publication status||Published - 2016|
|Event||The 2016 Meeting of the Animal Science Modelling Group - Salt Lake Marriott Downtown, Salt Lake City, United States|
Duration: 19 Jul 2016 → 19 Jul 2016