Effect of thyroidectomy on pregnenolone and progesterone biosynthesis in rat adrenal cortex

The effects of thyroidectomy on pregnenolone and progesterone biosynthesis were investigated in rat adrenal cortex mitochondria and microsomes. The sequential hydroxylations of cholesterol, and the latter's side-chain cleavage, which constitute the limiting step in steroidogenesis, were studied by measuring oxygen consumption rates in the presence of cholesterol derivatives. The addition of (20R)-hydroxycholesterol, (22R)-hydroxycholesterol or (20R,22R)-dihydroxycholesterol stimulated these rates, but addition of cholesterol or (22S)-hydroxycholesterol had little effect. Thyroidectomy significantly reduced oxygen consumption rates in the presence of the sterols by about 30%. Oxygen uptake was small in the presence of respiratory inhibitors; the addition of sterols raised this uptake but subsequent thyroidectomy did not change it. Rat adrenal cortex mitochondria and microsomes converted pregnenolone ino progesterone through 3 beta ol dehydrogenase/delta 4-5 isomerase, in two different successive steps. Values for enzyme activities were 0.18, 0.26 and 0.81 nmol progesterone/min/mg protein for the overall complex, the 3 beta ol dehydrogenase and the delta 4-5 isomerase respectively. All enzyme activities were unchanged by thyroidectomy. Similar results were obtained for corresponding microsomal activities whose values were in the same range. For both microsomes and mitochondria, the dehydrogenase reaction was the limiting step in the enzyme reaction leading to progesterone formation from pregnenolone. The limiting step in corticosteroidogenesis leading to prenenolone formation by an NADPH-dependent step was slowed down by thyroidectomy, probably because the reaction that transfers energy from NADH to NADP was inhibited. The enzyme complex leading to progesterone which involves NAD+ as cofactor, was unchanged by thyroidectomy. Thyroid hormones may therefore affect the availability of the energy mechanisms connected with the proton motive force, since thyroidectomy reduces both the phosphorylative oxidation and energy-dependent hydroxylation reactions involved in steroidogenesis.