Effects of thyroxine on the synthesis of cholesterol and fatty acids by cell‐free fractions of rat liver

Abstract

We have shown elsewhere (Fletcher & Myant, 1958) that the synthesis of cholesterol from acetate by rat liver slices is increased if the rat is given injections of thyroxine before the liver is removed. If mevalonic acid, instead of acetate, is used as the precursor of cholesterol, this effect is absent or only just detectable. When the daily dose of thyroxine is 20 ,ug or more, synthesis of fatty acids by the liver slices is depressed. It was suggested that treatment with thyroxine stimulates cholesterol synthesis by acting upon a rate-limiting step between acetate and mevalonic acid, and that the depression of fatty acid synthesis might be due to a deficiency of ATP.In this paper we describe experiments designed to test whether these effects of thyroxine can be localized in one or other of the fractions of the liver cell. Cholesterol (Bucher & McGarrahan, 1956) and fatty acids (Langdon, 1955;Seubert, Greull & Lynen, 1957) have been shown to be synthesized in vitro by homogenates of mammalian liver from which the heavier subcellular particles have been removed. We have therefore studied cholesterol and fatty acid synthesis in such preparations, obtained from the livers of thyroxine-treated and normal rats. METHODSAnimals. The rats were albino males of an inbred strain, weighing 120-200 g. The treated animals were given sodium-L-thyroxine (20-50 jg in 0-2 ml. water) daily for 9 days by intraperitoneal injection under light ether anaesthesia. The solutions of thyroxine were made by dissolving the required amount in a small quantity of water with the addition of one drop of 20 % NaOH, and then diluting to 100 ml. with water. Control animals were similarly injected with 0-2 ml. of an alkaline solution containing no thyroxine. During treatment with 20 or 30 ,g of thyroxine per day, the weight gain and food consumption of the treated rats did not differ significantly from that of the controls. When the daily dose of thyroxine was 50 Zg, the treated animals gained less weight than the controls.Preparation of tissue. At the end of the treatment, the animals were killed by breaking their necks. The livers were removed as rapidly as possible and placed on ice. Homogenates were then prepared from the pooled livers of each group of treated and control rats, 10PHYSIO. CLIV

Partial reversal of the effects of thyroxine on lipid synthesis in rat liver by the addition of cofactors in vitro

“…These observations are more detailed than those we have already reported (Fletcher & Myant, 1960), which were based only on the naked-eye appearance of the glycogen pellet obtained by centrifugation.…”

“…At the end of the incubation cholesterol as digitonide and fatty acids as soaps were isolated from the contents of the flasks and assayed for radio-activity by methods described elsewhere (Fletcher & Myant, 1960). The amounts of lipid synthesized were expressed as micromoles radioactive acetate incorporated into cholesterol or fatty acids per gram of fresh liver.…”

“…At the end of the period of treatment the rats were killed by a blow on the head and the livers removed. Cell-free fractions were prepared in phosphate buffer (Bucher & McGarrahan, 1956) by the methods described elsewhere (Fletcher & Myant, 1960).…”

Stimulation of Hepatic β-Hydroxy-β-methylglutaryl Coenzyme A Reductase Activity in Hypophysectomized Rats by L-Triiodothyronine

“…Any animal that gained weight as readily as normal animals was discarded. The animals were killed between 12 and 20 days after the operation.…”

“…Administration of insulin restored the activity to nearly normal levels within 2 hr. Other hormones that have been implicated in stimulating cholesterol biosynthesis are thyroid hormones (12,13) and norepinephrine (14). Guder et al (13) have shown that hepatic HMG-CoA reductase activity was decreased to less than 50% of normal in hypothyroid rats (produced by 13l1 treatment).…”