Thyrotropin alters the utilization of thyroglobulin's hormonogenic sites.

We injected rabbits and guinea pigs with bovine thyrotropin (TSH) daily for 3 days, while controls received saline. All animals received sodium [125I]iodide on the second day, and thyroglobulin was purified from the thyroids of each group by gel filtration. Hormonogenic tryptic peptides from each S-cyanoethylated thyroglobulin preparation were isolated by high performance liquid chromatography, and their amino acid sequences were determined, permitting their localization within the thyroglobulin polypeptide chain by comparison with cDNA-derived sequences from bovine and human thyroglobulins. Thyroglobulins from the saline-injected rabbits and guinea pigs contained the same four major hormonogenic sites, designated A-D, previously described (Dunn, J. T., Anderson, P. C., Fox, J. W., Fassler, C. A., Dunn, A. D., Hite, L. A., and Moore, R. C. (1987) J. Biol. Chem. 262, 16948-16952). In both species, sites A and C were the major loci for thyroxine and triiodothyronine, respectively. However, site D in the guinea pig had a greater ratio of [125I]thyroxine to [127I]thyroxine than did site A, whereas the reverse was true in the rabbit. TSH administration produced the following changes in thyroglobulins of both species, relative to controls: 1) an increase in the ratio of [125I]triiodothyronine to [125I] thyroxine (rabbit, 0.29 versus 0.17; guinea pig, 0.19 versus 0.08), with the increase in triiodothyronine principally at site C; 2) a marked increase in 125I/127I and in thyroxine formation at site D (14.1% of thyroglobulin's thyroxine versus 9.8% in rabbits, 24 versus 13% in guinea pigs); 3) a corresponding decrease in thyroxine formation at site A (33 versus 43% in rabbits, 30 versus 46% in guinea pigs); and 4) a sharp increase in conversion of thyroglobulin's N-terminal 125I-labeled approximately 20 kDa hormone-rich iodopeptide, which contains site A, to a 125I-labeled approximately 15-kDa (rabbit) or 125I-labeled approximately 13-kDa (guinea pig) form, reflecting probable peptide bond cleavage. Our results show that TSH alters both the structure of the thyroglobulin molecule and the priority of utilization of its hormonogenic sites. We conclude that these changes are important to TSH's enhancement of thyroid hormone synthesis.

The sites of thyroid hormone formation in rabbit thyroglobulin.

Rabbit thyroglobulin (Tg) was labeled in vivo with 125I and purified by gel filtration. Separation by high performance liquid chromatography (HPLC) of tryptic digests of S-cyanoethylated Tg yielded four major iodothyronine-containing peaks, designated A, B, C, and D. These were further purified on HPLC and sequenced for identification of amino acid residues and for location of the iodothyronine by 125I counting. The published primary structure for bovine Tg, derived from cDNA sequencing of the Tg gene (Mercken, L., Simons, M.J., Swillens, S., Massaer, M., and Vassart, G. (1985) Nature 316, 647-651), permitted tentative location of the rabbit hormonogenic peptides within the Tg polypeptide chain. Site A, corresponding to bovine residue 5, contained 44% of Tgs [125I]T4 (thyroxine) and 25% of its [125I]T3 (triiodothyronine); its specific activity of iodine was higher than that for other sites, indicating priority of iodination. Site B, containing 24% of Tgs [125I]T4 and 18% of its [125I]T3, corresponded to bovine residue 2555. Site C, at the third residue from the C terminus (bovine residue 2748), was the major T3 site, accounting for over 50% of Tgs [125I]T3. The amino acid sequence around this site shows less homology among different animal species than do those flanking the other hormonogenic sites. Site D accounted for 17% of Tgs [125I]T4 and corresponded to bovine Tyr-1291, in the midportion of Tgs polypeptide chain. The three major T4-forming sites had the sequence Asp-Tyr (sites B and D) or Glu-Tyr (site A), while the sequence Ser-Tyr-Ser appeared to favor T3 synthesis (site C), suggesting an important influence of primary structure on hormonogenesis. We conclude that site A is the major T4-forming site and site C the major T3-forming one, but others are available and offer the opportunity for flexibility in meeting different demands for hormone formation.

Magnesium toxicity as a cause of hypotension and hypoventilation. Occurrence in patients with normal renal function.

Symptomatic hypermagnesemia usually requires both increased intake of the ion and abnormal renal function; however, we treated two patients with iatrogenic hypermagnesemia (10.4 and 13.2 mEg/L) who had normal renal function. One received ureteral irrigation with hemiacidrin (Renacidin) to dissolve a stone, and the other was treated for ingestion of an unknown toxin with large doses of magnesium sulfate. Therapy included ventilatory support, intravenous calcium, and fluids. Dialysis was not required, and recovery was complete.