Effect of L‐triiodothyronine on Δ9 desaturase activity in liver microsomes of male rats

Male rats injected with a single saturating dose of L‐triiodothyronine (T3) showed, after a lag time of approximately eight hr, a sharp rise in Δ9 desaturase activity. Desaturase activity reached a plateau which was 1–1.2 times above the base line levels of rats which were not hormone‐treated. The plateau was maintained for five days in animals which were kept on daily hormone‐treatment. The increase in Δ9 desaturase activity by T3 required ongoing protein synthesis, because the increase in enzymatic activity due to hormone treatment was completely abolished in the presence of cycloheximide. These findings suggest that cycloheximide may block the induction of Δ9 desaturase by T3 and/or inhibit the synthesis of protein(s) essential to the desaturation‐response to T3. Modifications observed in liver microsomal fatty acid composition in T3 treated rats were independent of the effect on desaturation. It is suggested that other factors, such as diet, membrane lipid synthesis and degradation, as well as fatty acid turnover and oxidation, could be involved in affecting the fatty acid composition of thyroid hormone‐treated rats.