Thyroid Control over Biomembranes

Hypothyroid rats were prepared by thyroidectomy and maintenance on a low‐iodine diet (group A); Group B was additionally pretreated with 0.5 mCi of 131 I as NaI, given intraperitoneally. Liver microsomes obtained from hypothyroid and normal rats were compared. After fasting and re‐feeding on 20% sucrose solution, high levels of microsomal fatty‐acyl‐CoA Δ 9 ‐desaturase (as measured spectrophotometrically by the rate constants for cytochrome b 5 reoxidation) were induced in all the normal animals, half of the group A hypothyroid rats, and none of the group B hypothyroid rats. Hypothyroidism did not change desaturase Arrhenius profiles or V and K m for NADH–cytochrome c reductase, but increased content of cytochrome b 5 . The inability of adequately hypothyroid rats to induce the Δ 9 ‐desaturase seems to be specific, in that injection of methylcholanthrene successfully induced microsomal benzpyrene monooxygenase activity and increased cytochrome b 5 contents in hypothyroid animals. The defects in overall fatty acyl desaturation reported in hypothyroid animals [Landriscina, C., Gnoni, G. V. & Quagliariello, E. (1976) Eur. J. Biochem. 71 , 135–143] are suggested to be due to deficiencies in the specific desaturase(s).