Alteration of aminoacyl-tRNA synthetase with age: heat-labilization of the enzyme by oxidative damage.

Active oxygens have been suggested to be involved in age-related alterations of organelles and molecules. In this study we investigated the influence of active oxygen on aminoacyl-tRNA synthetases partially purified from rat liver. Treatment of leucyl-tRNA synthetase with Fe3(+)-ascorbate resulted in the increased heat-lability of the enzyme. The inactivation was inhibited by radical scavengers such as mannitol and benzoate, suggesting that hydroxyl radicals are responsible for heat-labilization of the enzyme. On the other hand, a considerable part of tyrosyl-tRNA synthetase was converted to heat-labile forms without added iron and ascorbate under aerobic conditions but not under anaerobic conditions. These and other findings suggested that the heat-labilization of this enzyme is caused by active oxygens probably generated by the reaction of dioxygen and transition metal ions present in the enzyme preparations. Heat-inactivation curves of the enzymes modified as described above were similar to those observed for the enzymes from aged animals in that these enzymes exhibited higher percentages of heat-labile forms than the unmodified enzymes from young animals [Takahashi and Goto, 1987, Arch. Gerontol. Geriatr. 6, 73-82; Takahashi and Goto, 1987, Arch. Biochem. Biophys. 257, 200-206]. The present findings are consistent with the theory that active oxygens are involved in the age-related alterations of enzymes.