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Peculiarities of tyrosine metabolism in the rat liver under the condition of protein deficiency

In the present study, a tyrosine content in liver and enzymatic activities of its metabolism: tyrosine aminotransferase, 4-hydroxyphenylpyruvate dioxygenase, aldehyde dehydrogenase (ALDH3A1) were investigated under the conditions of alimentary protein deprivation. The experiments were performed on white rats with a body weight of 100-150 g aging 2.5-3 months. The animals were divided into two experimental groups: I – animals receiving a full-value semi-synthetic ration (C); II – animals receiving a low-protein ration (LPR). In order to simulate the low-protein diet, the animals received a diet containing 4.7% protein, 10% fat and 85.3% carbohydrates for 28 days, calculated according to the American Institute of Nutrition's recommendations. Determination of tyrosine in deproteinized by 6% sulfosalicylic acid extracts of the liver tissue was performed using the automatic analyzer of amino acids T-339 (Microtechnology, Czech Republic). The enzyme activity was determined by spectrophotometric method – tyrosine aminotransferase by the amount of 4-hydroxybenzaldehyde, which has a maximum absorption at 331 nm, 4-hydroxyphenylpyruvate dioxygenase – by the intensity of colored product formation at λ 336 nm, ALDH3A1 activity was measured at 340 nm wavelength. It was established a 5-fold depletion of the tyrosine pool and a 2-fold reduction of the tyrosine aminotransferase activity, which catalyzes the formation of the first transformation of the tyrosine – 4-hydroxyphenylpyruvate transformation in the liver tissue under the condition of protein deficiency. Determination of the key enzymes activity of two possible ways of further 4-hydroxyphenylpyruvate transformation has shown that the activity of 4-hydroxyphenylpyruvate dioxygenase, as an enzyme of the homogentisin pathway of tyrosine metabolism, remains at the control level, while the aldehyde dehydrogenase, as the key enzyme of the synthesis pathway of the benzoyl ring of ubiquinone molecule, is by half reduced. The obtained results allow us to conclude that under the conditions of an alimentary protein deficiency tyrosine is predominantly metabolized in the liver by the homogenetistic pathway, which can be considered as a compensatory reaction directed at the maintaining energy metabolism while simultaneously affecting the use of tyrosine as a precursor in ubiquinone synthesis. 
 Key words: alimentary protein deficiency, liver, mitochondria, tyrosine, ubiquinone, tyrosine aminotransferase, 4-hydroxyphenylpyruvate dioxygenase, aldehyde dehydrogenase