Phenylalanyl-tRNA synthetases of rat liver: differential effects of thyroid hormone

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Abstract
Thyroxine and analogues inhibit rat liver aminoacyl-tRNA synthetase activity for phenylalanine and tyrosine. A high yield purification of the major cytoplasmic form of phenylalanyl-tRNA synthetase (C1) and its characterization is reported. Polyribosome-bound and other sedimentable forms are found to be indistinguishable from soluble enzyme by immunoprecipitation. Mitochondrial phenylalanyl-tRNA synthetase (M) and cytoplasmic activity (C2) resistant to anti-C1 antibody have been partially purified and characterized. Tissue levels of the three forms are estimated at 22, 1.8, and 4.1 units/g of liver for C1, C2, and M, respectively [1 unit = 1 nmol of Phe-tRNA/min, 30 degrees C]. Charging capability toward rat liver and yeast tRNA, kinetic parameters, and physical properties are compared. Only enzyme C1 is hormone inhibited [K1 = 4 x 10(-6) M for triiodothyronine]. The data indicata that C2 and M are not structurally related to C1; C2 may represent an independent cytoplasmic pool of M. Implications of C1 inhibition in relation to effects on liver protein synthesis are discussed.

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