The nature of particulate thyroid proteins in an experimental rat thyroid tumor

Abstract

Particulate proteins from microsomes have been studied in a rat thyroid tumor. The tumors were labeled in vivo with intraperitoneal injection of [ 14C]leucine or 125I-. A large proportion (20–50%) of the radioactivity, either 14C or 125I-, was particulate and associated with the nuclear or microsomal pellets. Proteins released by deoxycholate from microsomes were analyzed by density gradient centrifugation and tested with a rabbit anti-rat thyroglobulin serum. It was found that 60% of the proteins labeled with 125I and 30% of those labeled with 14C had a sedimentation rate of 12 S. The 12-S fractions reacted with anti-thyroglobulin serum. Density gradient centrifugation patterns of soluble proteins labeled with 14C or 125I for 30 min-18 h showed that 125I is incorporated into 19-S thyroglobulin, whereas no more than 10% of [ 14C]leucine was found in the 19-S protein. The remaining 80–90% of the radioactivity had a sedimentation rate of 3–8 S. We conclude that in this tumor a large proportion of thyroglobulin or its precursors is in the microsomes, whereas soluble thyroglobulin is low or absent possibly due to a deficiency in the mechanism for release of protein from endoplasmic reticulum.