Biosynthesis and turnover of a microsomal protein disulfide isomerase in rat liver.

The biosynthesis and turnover of one type of microsomal protein disulfide isomerase (PDI) in rat liver were studied. The enzyme is predominantly synthesized by the membrane-bound ribosomes of rough endoplasmic reticulum, as judged from the results of immunological analyses of puromycin-released nascent peptides and in vitro translation products of isolated free and bound polyribosomes. Nascent peptides of PDI were released from rough microsomes by treatment with puromycin in the presence of 0.5 M KCI, which indicates that the nascent peptides of this enzyme are released on the outer surface of microsomal vesicles. This enzyme accounts for about 0.1 to 0.2% of the total protein synthesis by membrane-bound ribosomes. The half-life of PDI was about 7 days, which was significantly longer than the average half-life of microsomal proteins.