The amount of
elongation factor 2 (EF-2) associated with different ribosomal fractions (mono- and polyribosomes) isolated from a
methylcholanthrene-induced
sarcoma is estimated during
tumor growth (exponential and plateau phase of growth). Direct
EF-2 quantification is obtained by a modification of the method of the
diphtheria toxin-catalyzed transfer of (14C)ADP-ribose from (14C)NAD+ to the
enzyme. Data reported show that the amount of
EF-2 associated with the monoribosomal fraction changes during
tumor growth, and particularly, that this amount increases when the
tumor cells enter into the plateau phase. In contrast, the
EF-2 content of the polyribosomal fraction does not change during the different phases of
tumor growth. Data also show that the amount of
EF-2 bound to the monoribosomal fraction isolated from
tumor cells is significantly and constantly lower than that of the corresponding fraction isolated from reticulocytes or hepatocytes. Moreover the
tumor monoribosomes generated by the polyribosome breakdown induced by the "
starvation" procedure did not show significant changes in their
EF-2 content with respect to monoribosomes isolated from
tumor cells maintained in physiological conditions. Besides,
tumor monoribosomes generated by the polyribosome breakdown induced by
puromycin or by running-off treatment exhibit a relevant increase of the
EF-2 content. In these conditions the amount of
EF-2 associated with the monoribosomes is similar to that associated with the monoribosomes of control cells (hepatocytes and reticulocytes). Results are discussed in view of a possible regulative role of the
EF-2 enzyme in the ribosomal cycle of eukaryotic cells.