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Superoxide dismutase was covalently coupled wih aldehyde dextran, a polymeric carrier of molecular mass of 70 kDa. Modification produced an increase in the enzyme thermostability. Modified preparations retained a high specific activity. The composition of the thus obtained conjugates was analyzed by the ultracentrifugation and diffusion methods. The protein induced the destruction of aldehyde dextran, the enzyme being modified by its fragments. The presence of aldehyde dextran excess in the incubation medium promoted superoxide dismutase dissociation into individual subunits. At the enzyme/carrier ratio of 1:02 modification occurred as covalent coupling of the biocatalyst subunits and its one-point modification.

A procedure for preparation of samples is described for emission-spectroscopic estimation of 15N in ammonia, glutamic aspartic acids, leucine and glycine using 15N-analyzer NOI-5. The assay involved separation of ammonia and amino acids by means of ion exchange chromatography, degradation of amino acids to ammonia its isolation by microdiffusion from buffers and subsequent reduction of ammonia to nitrogen. Amount of nitrogen in the samples was about 20 mg. The procedure enabled to estimate the content of 15N in ammonia and amino acids of physiological fluids.

Urea synthesis was studied in the isolated rat heart-perfused with ammonium chloride (10 mM), mixtures of ammonium chloride (10 mM) and 1-aspartic acid (10 mM), 1-ornithine (2.5 mM), 1-arginine (10mM), 1-glutamine (10 mM), 1-alanine (10 mM), 1-leucine (5 mM) and pyruvate (5mM). Ammonium chloride and 1-arginine are the most effective activators of urea synthesis, while 1-leucine and pyruvate produce an inhibitory action. The possibility of ammonia hinding in heart tissue was shown by means of (15)N. The amount of urea incorporated (15)N grows as the concentration of (15)N-ammonium in the perfusate increases from 1.6 to 3.4 mM. Isoproterenol-induced necrosis of the heart muscle leads to a significant increase in the urea incorporated (15)N.

The pathways of ammonia neutralization in the rat heart muscle were traced with nitrogen-15 isotope. The effect of ammonia concentration in the perfusate on its utilization by the myocardium was investigated over the range of 1.6--3.4 mM. Considerable amounts of the neutralized ammonia were found in glutamine and glutamic acid, less quantities in alanine and aspartic acid. Isoproterenol-induced necrosis of the heart muscle leads to a decrease in both glutamine and alanine ability to bind ammonia (20--40 and 30--45%, respectively). The necrosis effect is not significant for glutamic acid. In necrosis nitrogen-15 incorporation into aspartic acid declines whereas with higher concentrations it significantly increases.