With 125I-labeled Fab' specific for rat liver
serine dehydratase it has been possible to localize polyribosomes synthesizing the
enzyme under several different environmental conditions. Evidence is presented to show that, following the administration of
amino acids in vivo, the relative synthetic capabilities of free and membrane-bound polyribosomes synthesizing
serine dehydratase vary with time. Early during the period of induction of the
enzyme by administration of
amino acids or by feeding a
high protein diet the majority of the newly synthesized
enzyme is derived from membrane-bound polyribosomes. Later in the induction process an increasing proportion of the
enzyme is synthesized by the free polyribosomes. Subcellular localization studies clearly show that
serine dehydratase is synthesized by both subclasses of hepatic membrane-bound polyribosomes, the loose and tight membrane-bound polyribosomes, as well as by the free polyribosomes. It was found that the membrane-bound polyribosomes are the preferential sites of synthesis of the majority of
serine dehydratase molecules in the Morris
hepatomas 5123C and 7800. It is concluded that the synthesis of the
enzyme,
serine dehydratase, in rat liver is not discretely compartmentalized in either class of free or membrane-bound polyribosomes. Rather, the relative proportions of the
serine dehydratase synthesizing polyribosomes within these two classes of polyribosomes can vary depending on the metabolic and physiologic state of the liver cell.