Effect of cycloheximide on iodothyronine formation in vitro

The unique inhibitory effect of cycloheximide (CH) on the coupling of iodotyrosines was examined in vitro. Rat thyroid lobes were incubated for 8 h under our improved condition. In the presence of 10(-4) - 10(-3) M CH, the per cent uptake of 131I decreased, proportionate synthesis of [131I]MIT increased slightly, and that of [131I]T4 or [131I]T3 decreased markedly. The incorporation of medium 127I into T4 or T3 during the 8 h incubation period decreased markedly, but was fairly constant into MIT and only slightly decreased into DIT. Thus the inhibitory effect of CH seemed more prominent on iodothyronine formation than on iodotyrosine formation in this in vitro system. Inhibition of formation of newly labelled iodothyronines seemed to occur almost in parallel with the inhibition of [3H]amino acid incorporation into the thyroidal soluble protein. However, the coupling of iodotyrosines prelabelled in the absence of CH did not seem to be affected by CH. The presence of 10(-4) M CH induced the Wolff-Chaikoff effect at a lower iodide concentration than that which occurred in the absence of CH, suggesting that CH sensitized the Wolff-Chaikoff effect. However, the organification of 127I and T4 synthesis were markedly reduced in the presence of CH even before the apparent Wolff-Chaikoff effect was initiated. These results give further support to out contention that prethyroglobulin is more important for organification of iodide than pre-existing thyroglobulin. We conclude that CH reduces coupling efficiency indirectly, probably by inhibiting the formation of prethyroglobulin with a favourable structure for coupling.