Dependency on the source of supernatant factors for optimal 14C-polyphenylalanine synthesis by high-salt wash treated liver ribosomal subunits in rats of different ages

The rate of poly-U translation and polyphenylalanine formation by liver cell free systems including 14C-Phe- tRNA, poly-U, GTP, 3· m m MG 4 + in the presence of 0·5 m KCl washed 40S and 60S ribosomal subunits (1 : 2) and Sephadex G-25 treated supernatant factors (S-100 fraction) has been investigated in young adult (6 month), middle-aged (13–14 month) and old (24–26 month) male Wistar rats. When homologous cell free systems are tested those made by subunits and S-100 fraction from middle-aged rats appear to synthesize more polyphenylalanine than systems of young-adult and old rats. Old rat systems, however, appear to synthesize more polyphenylalanine than young-adult ones. When heterologous young-adult: old subunit: S-100 systems are tested the synthesis of polyphenylalanine is impaired. It is restored to normal by employing the S-100 fraction of middle-aged rats. These observations suggest the presence of modified initiation and/or elongation factor activities in the liver cell sap fractions of young-adult and old normal rats. These modifications could be the expression of a general regulatory mechanism of protein biosynthesis at these age periods. The possibility of ribosomal alterations in the conditions studied appears to be ruled out.