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A Minimal Compartmental Model for the Assessment of Net Whole Body Protein Breakdown, Using a Pulse of Phenylalanine and Tyrosine Stable Isotopes in Humans
Author(s) -
Mason Alvise,
Engelen Marielle,
Toffolo Gianna,
Deutz Nicolaas
Publication year - 2016
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.30.1_supplement.909.10
Subject(s) - phenylalanine , tyrosine , chemistry , kinetics , tracer , isotope , amino acid , hydroxylation , pulse (music) , stable isotope ratio , phenylalanine hydroxylase , radiochemistry , chromatography , biochemistry , enzyme , physics , quantum mechanics , detector , nuclear physics , electrical engineering , engineering
Rationale We recently developed a novel and easy‐to‐use approach to measure net whole body protein breakdown (netPB), using non‐compartmental modelling after single pulse injection of the stable isotopes phenylalanine (PHE) and tyrosine (TYR). We further fine‐tuned this approach by developing a minimal compartmental model to add structural information to amino acids kinetics in PB with high precision. Method Healthy human subjects (8 male, 6 female, age: 59.6 ± 9.0 years) were given in the post absorptive state a single pulse (8ml) injection of L‐[ring‐13C6]PHE (6.44 mg/ml) and L[ring‐2H4]TYR (0.46 mg/ml). Multiple plasma samples were collected for 120 min and tracer‐tracee ratio of PHE (mass 6) and TYR (mass 4 and 6) were measured by LC‐MS/MS. A four compartment model was developed to describe the kinetics of PHE and TYR and the PHE to TYR hydroxylation. The model is a priori identifiable from the PHE‐6 and TYR‐6 data measured following the PHE‐6 injection, provided that the TYR‐6 accessible pool size is known. Since this pool represents plasma and rapidly equilibrating tissues (mainly the liver) where the PHE to TYR interconversion takes place, its pool size was assumed to be equal to 25% of the whole body TYR pool size, estimated from the TYR‐4 decay curve. The model, identified by using SAAM, was able to reproduce the experimental data of all individuals, and all its parameters were estimated with high precision (VC%: 14.78 ± 7.77). Our model provides estimates of compartmental fluxes, including the plasma appearance of PHE and TYR, and the PHE to TYR interconversion. These fluxes were compared to their counterparts, estimated via non‐compartmental analysis in order to assess the physiological relevance of the estimated variables. Results The following data were obtained by non‐compartmental versus compartmental analysis for Net‐PB (6.998 ± 2.237 umol/FFM kg/h, versus 6.475 ± 1.899 umol/FFM kg/h, resp.), rate of appearance of PHE (53.626 ± 7.479 umol/FFM kg/h versus 49.675 ± 5.137 umol/FFM kg/h, resp.), rate of appearance of TYR in plasma (44.686 ± 9.012 umol/FFM kg/h versus 45.879 ± 9.110 umol/FFM kg/h, resp.). These data suggest that our model is physiologically meaningful. Conclusion Our data reveal that after single pulse injection of PHE and TYR stable isotopes, a minimal compartmental model analysis estimates net PB with low variability when the physiology of PHE to TYR conversion is included in the model estimations. Furthermore, our model may provide more detailed insight in the metabolism of these amino acids.