Optimality in the zonation of ammonia detoxification in rodent liver
The rodent liver eliminates toxic ammonia. In mammals, three enzymes (or enzyme systems) are involved in this process: glutaminase, glutamine synthetase and the urea cycle enzymes, represented by carbamoyl phosphate synthetase. The distribution of these enzymes for optimal ammonia detoxification was determined by numerical optimization. This in silico approach predicted that the enzymes have to be zonated in order to achieve maximal removal of toxic ammonia and minimal changes in glutamine concentration. Using 13 compartments, representing hepatocytes, the following predictions were generated: glutamine synthetase is active only within a narrow pericentral zone. Glutaminase and carbamoyl phosphate synthetase are located in the periportal zone in a non-homogeneous distribution. This correlates well with the paradoxical observation that in a first step glutamine-bound ammonia is released (by glutaminase) although one of the functions of the liver is detoxification by ammonia fixation. The in silico approach correctly predicted the in vivo enzyme distributions also for non-physiological conditions (e.g. starvation) and during regeneration after tetrachloromethane (CCl4) intoxication. Metabolite concentrations of glutamine, ammonia and urea in each compartment, representing individual hepatocytes, were predicted. Finally, a sensitivity analysis showed a striking robustness of the results. These bioinformatics predictions were validated experimentally by immunohistochemistry and are supported by the literature. In summary, optimization approaches like the one applied can provide valuable explanations and high-quality predictions for in vivo enzyme and metabolite distributions in tissues and can reveal unknown metabolic functions.
KeywordsLiver zonation Ammonia metabolism Glutamine synthetase Optimization Systems biology
Carbamoyl phosphate synthetase
Parts of this work have been supported by the German Virtual Liver Initiative (www.virtual-liver.de) of the German Federal Ministry of Education and Research (RG: 0315735, DD and SH: 0315760 and CK: 0315758) and the German Research Foundation (CK: KA 3541/3-1). We thank J. Schleicher, Ch. Tokarski and S. Vlaic for stimulating discussions.
MB, MP prepared and conducted the optimization. AG, JH prepared and conducted the experimental validation. MB, MP, SZ, PL conceived the main part of research. MB, SZ wrote the main part of manuscript. MP, AG, DD, SH, JH, SS, CK, RG involved in discussion and interpretation of results as well as writing parts of the manuscript.
Compliance with ethical standards
All procedures were in accordance with the ethical standards of the institution.
Conflict of interest
The authors declare that they have no conflict of interests.
- Comar JF, Suzuki-Kemmelmeier F, Nascimento ÉA, Bracht A (2007) Flexibility of the hepatic zonation of carbon and nitrogen fluxes linked to lactate and pyruvate transformations in the presence of ammonia. Am J Physiol Gastrointest Liver Physiol 293(4):G838–G849. doi: 10.1152/ajpgi.00120.2007 CrossRefPubMedGoogle Scholar
- Ghallab AM (2013) Spatial-temporal modelling of liver damage as well as regeneration and its influence on metabolic liver function. Thesis, Justus-Liebig-Universität GießenGoogle Scholar
- Toepfer S, Zellmer S, Driesch D, et al (2007) Compartment model of glutamine and ammonia metabolism in liver tissue. In: Scharff P (ed) Proceedings of the 52nd international scientific colloquium, Ilmenau, Germany, vol 2, pp 107–112Google Scholar
- Ueberham E, Arendt E, Starke M, Bittner R, Gebhardt R (2004) Reduction and expansion of the glutamine synthetase expressing zone in livers from tetracycline controlled TGF-β1 transgenic mice and multiple starved mice. J Hepatol 41(1):75–81. doi: 10.1016/j.jhep.2004.03.024 CrossRefPubMedGoogle Scholar
- Zellmer S, Toepfer S, Driesch D, et al (2007) Modelling of hepatic glutamine metabolism. In: Allgöwer F, Reuss M (eds) Proceedings of the 2nd conference foundations of systems biology in engineering, Stuttgart, pp 183–187Google Scholar