Optimality in the zonation of ammonia detoxification in rodent liver
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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.
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