Effect of Thyroxine on Δ6 and Δ9 Desaturation Activity

Many studies have revealed that thyroid hormones markedly affect lipid metabolism in man and in several species of animals. Concerning fatty acid biosynthesis it was demonstrated that the administration of thyroxine stimulates the incorporation of l–14c acetate into fatty acids in rats and mice (Dayton et al, 1960) (Gompertz and Greenbaum, 1966) (Marchi and Mayer, 1959). According to Gompertz and Greenbaum (1966) these observations appear to be associated with an increase of stearyl-CoA desaturase activity. Moreover Myant and Iliffe (1963) found that rats treated with thyroxine showed an inhibition of acetate incorporation, but not of malonate incorporation, into fatty acids by mitochondria free, subcellular liver preparations. Other authors have shown that the thyrotoxic state was accompanied by an increased incorporation of acetyl-CoA to fatty acid and a rise in the activity of fatty acid synthetase in rat livers (Diamant et al, 1972) (Roncari and Murthy, 1975). However, in vitro studies of fatty acid synthesis in which liver supernatant of 105,000 xg and microsomal preparations were incubated with the hormone showed that thyroxine inhibits de novo synthesis of palmitate and stimulates the desaturation reactions (Faas et al, 1972).