STUDIES ON PROTEIN SYNTHESIS IN THYROID .3. IN VIVO INCORPORATION OF LEUCINE-3H INTO THYROGLOBULIN OF MICROSOMAL SUBFRACTIONS OF RAT THYROID

Rats were given leucine-3H at different intervals, ranging from 30 minutes to 3 hours, before sacrifice. The thyroids were homogenized, and one mitochondrial fraction and four microsomal subfractions were prepared. The composition of the subfractions was studied electron microscopically and chemically. Distinct patterns of protein labeling appeared only in two microsomal subfractions. One of these, consisting of ribosome-studded vesicles with a rather dense content, shows labeling of 3-8 S and 12 S proteins as well as thyroglobulin already at 30 minutes. At 1 hour there is a very high labeling of 3-8 S and 12 S proteins and a pronounced radioactivity in thyroglobulin. At 3 hours the thyroglobulin labeling is very high in this subfraction and exceeds that of 12 S. The other microsomal subfraction containing labeled material is composed of smooth-surfaced vesicles of various size and density. This fraction shows no labeled components at 30 minutes, but at 1 hour it displays two radioactivity peaks, one corresponding to thyroglobulin and the other, lower, to 12 S protein. At 3 hours labeling is found only in thyroglobulin. The results strongly indicate that the microsomal subfraction consisting of ribosome-studded vesicles is alone responsible for the thyroglobulin synthesizing capacity. The presence of labeled thyroglobulin in the subfraction consisting of smooth-surfaced vesicles is thought to represent in vivo transfer of thyroglobulin from the endoplasmic reticulum to the follicle lumen. © 1968 Academic Press Inc.