[46] Aminoacyl-tRNA Synthetase Complex from Rat Liver

Aminoacyl-tRNA synthetases purified from a variety of sources generally have molecular weights between 70,000 and 200,000. This chapter describes the procedure that leads to the isolation of a high molecular weight complex containing aminoacyl-tRNA synthetases and most of the cellular tRNA. Under certain circumstances, only partial complexes are isolated. Some of the less tightly bound synthetases are easily removed from the complex. The assay measures the conversion of radioactive, acid-soluble amino acids into an acid-insoluble product due to their attachment to tRNA. The aminoacyl-tRNA synthetase complex is extremely unstable and is disrupted by a variety of treatments. More vigorous homogenization of liver using a motor-driven homogenizer leads to dissociation of the complex. The effect of excess homogenization is thought not to be on the complex directly, as it is too small, but is probably due to disruption of other structures, such as lysosomes that could release enzymes that act on components of the complex. The process of freezing and thawing destroys the aminoacyl-tRNA synthetase complex.