Age-associated accumulation of heat-labile aminoacyl-tRNA synthetases in mice and rats.

Age-related changes of two aminoacyl-tRNA synthetases in BDF1 mouse and Fischer 344 rat tissues were investigated. The proportions of heat-labile enzymes in the brain and liver were 5 to 10% in young animals, but 25 to 40% in old animals. The proportions began to increase markedly with increase in the mortality rate of the animals. Significant correlations were found between the proportions of heat-labile tyrosyl- and leucyl-tRNA synthetases in the brain and liver of individual animals. Moreover, the proportions of the heat-labile enzymes in the brain were also significantly correlated with those in the liver. Thus, it appears that when one enzyme is heat-labile in one tissue other enzymes are also heat-labile in the same tissue and the same enzyme is also heat-labile in other tissues. Analysis of the size-distribution of partially purified leucyl-tRNA synthetase complexes indicated that, while enzyme preparations from young animals tended to consist of complexes of larger sizes, those from older animals contained smaller complexes, a 10S complex being a major component. On heating preparations from old animals, the activity in the 10S peak disappeared most rapidly. This finding suggests that the heat-stability of the enzyme depended on its molecular form.