Thyroglobulins of cyclostomes and an elasmobranch

Abstract

As part of a study of the biosynthesis and physicochemical properties of primitive vertebrate thyroglobulins, the thyroid-containing tissues of Pacific hagfish ( Eptatretus stouti) and of Pacific lamprey ( Entosphenus tridentatus), and thyroid glands of Pacific dogfish ( Squalus suckleyi) were used. In the hagfish thyroid tissues labeled with radioiodine, only 3–8S iodoprotein having an approximate molecular weight of 155,000 was detectable. A low proportion of thyroid iodine was in the form of labeled thyroxine (2.5%) 4 days after labeling. In lamprey thyroid tissue radioactivity peaks corresponding to 3–8S and 19S proteins were found in ultracentrifugation patterns. However, the main thyroid iodoprotein component purified by ammonium sulfate fractional precipitation was 12S. The proportion of labeled thyroxine to total radioiodine in lamprey iodoproteins was 2.9%. Total iodine content of lamprey thyroprotein was 0.0022%. In dogfish thyroid glands only 19S thyroglobulin as obtained after purification by the ammonium sulfate procedure. Total iodine content was similar to that of mammalian thyroglobulins, 0.50%. The proportion of total iodine in the form of unlabeled thyroxine in an analysis of unlabeled dogfish thyroglobulin was at a high level of 50.9%. The amino acid compositions of lamprey iodoproteins and dogfish thyroglobulin differed in some respects from higher vertebrate thyroglobulins. There was also some difference in the carbohydrate compositions of lamprey iodoproteins and dogfish thyroglobulin, compared to higher vertebrate thyroglobulins. Moreover, the total carbohydrate content of lamprey iodoproteins and of dogfish thyroglobulin was lower than in higher vertebrate thyroglobulins. On the basis of these results the general properties of thyroglobulins could be considered with respect to the question of the evolution of thyroid function.