Biosynthesis of sulfite oxidase in rat liver-determination of the site of synthesis.

The site of biosynthesis of sulfite oxidase in rat liver was determined by immunoprecipitation of nascent peptides released from ribosomes and in vitro translation products of isolated polysomes. The specificity of the immunoprecipitation of sulfite oxidase nascent peptides was confirmed by examination of the size distribution of immunoprecipitated peptides and by the competition with purified enzyme in the immunoprecipitation. Both nascent peptides analysis and the in vitro translation experiment indicated that this enzyme was mainly synthesized by free ribosomes in the cytoplasm, although bound ribosomes also contributed to the synthesis of this enzyme to some extent. When the discharge of sulfite oxidase peptides from bound ribosomes was examined with rough microsomes, however, about two thirds of the enzyme peptides synthetized by bound ribosomes were discharged to the outside of microsomal vesicles.