Aminoacyl-tRNA synthetases (ARS) in consideration of the key role that they play in the process of protein biosynthesis, have caused specialinterest. They have been isolated and their properties have been tested in samples from plants, bacteria and animals. Aside from their differences in structure and form, they fulfil their function as specific aminoacylation tRNA. ARS in the cell are found mainly in the cytosol, but are also found in mitochondria. Until now extra-cellular ARS have not been observed. In the last few years however, there have been reports of ARS antibodies in the blood serum of patients with certain autoagressive diseases. These reasons resulted in the conduct of tests with the following goals: 1. the attempt to find extra-cellular ARS 2. analysing the possibility of liberating ARS from the cell 3. explaining the lack of ARS enzymatic activity in blood serum. Initially ARS activity in human physiological and pathological (gross organ damage) blood serums was tested. No enzymatic activity was observed in either complete blood serum or in their fractions. The lack of enzyme activity did not necessarily indicate that ARS was not present in blood serum. Tests were conducted in order to determine the presence of ARS by immunological methods. ARS samples were obtained from human tissues taken of a surgical clinic and then used to immunize rabbits. The serum thereby obtained contained antibodies that reacted with the fraction of free ARS and the macromollecular complex fraction. By using the technics of immunodiffusion and Immunoelectrophoresis the presence of ARS was determined in the blood serum of healthy persons as well as in those with higher enzyme activities. However the lack of ARS activity in blood serum was still in question. Further tests were conducted in order to answer this question using prepared hepatic cells. This cell suspension was obtained by perfusing rat liver with EDTA solution. These morphologically complete cells were then incubated in Krebs-Ringer solution. At certain incubation periods ARS activity was tested in extra-cellular fluid as well as in the cells. ARS activity as well as other enzyme activities (ASPAT, ALAT) were observed in extracellular fluid. This indicated that these enzymes exited the cells. Extra cellular activity of ARS was relatively low. Samples of ARS witii activities that were significantly higher than in the initial extracellular fluid were obtained from salted-out fractions. Simultaneously ARS activity was tested in complete, non-fractioned cell cytosol. It so happened that in this instance these enzymes had no activity. This indicated that perhaps the lack of ARS activity could be caused by some factor present in the cytosol as well as in blood serum that blocked their activity. The performance of fractioning of cytosol and blood serum showed that the fraction salted out by using over 70% ammonium sulphate was responsible for the process of ARS inactiva- tion. A closer analysis of this fraction showed that a protein with responsible for the inhibition function. This protein is present in both cytosol and blood serum and in both instances has similar quantities of amino acids. Aminoacylation is a two step and multicomponent reaction. Tests were conducted in order to determine the site where the aminoacylation inhibitor acted. These experiments showed that this inhibitor reacts with the ARS itself. It was also observed that the presence of strong protease inhibitors liquidates the inhibition effect of the tested protein. This fact, along with other observations, suggested that the ARS inhibitor which had been obtained may be a specific protease that damages the structure of these enzymes only. This idea is supported by knowledge of the fact that a natural ARS inhibitor is present in plants. The tests conducted below confirm that ARS is present in human blood serum as a result of passage from damaged tissues. They also confirmed their lack of activity in blood serum.
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