title: Phenylalanyl-tRNA synthetases of rat liver: differential effects of thyroid hormone.
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.
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000378:metabolism*; D000818:Animals; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D008930:Mitochondria, Liver / Q000187:drug effects / Q000201:enzymology; D010652:Phenylalanine-tRNA Ligase / Q000302:isolation & purification / Q000378:metabolism*; D051381:Rats; D013974:Thyroxine / Q000031:analogs & derivatives / Q000494:pharmacology*; D014284:Triiodothyronine / Q000494:pharmacology
chemical_list: D014284:Triiodothyronine; D000604:Amino Acyl-tRNA Synthetases; D010652:Phenylalanine-tRNA Ligase; D013974:Thyroxine