Regulation of palmitoyl-CoA chain elongation and linoleoyl-CoA chain elongation in rat liver microsomes and the differential effects of peroxisome proliferators, insulin and thyroid hormone.

Treatment of rats with p-chlorophenoxyisobutyric acid (clofibric acid), 2,2'-(decamethylenedithio)diethanol, di(2-ethylhexyl)phthalate or acetylsalicylic acid caused an increase in activity of palmitoyl-CoA chain elongation in hepatic microsomes. The activity of palmitoyl-CoA chain elongation decreased in both hypothyroid-state and diabetic-state rats, increased in hyperthyroid-state rats and did not change in adrenalectomized rats. The administration of clofibric acid to these rats in an altered hormonal state caused an increase in the activity of palmitoyl-CoA chain elongation, but no additional increase in the activity was observed with treatment of hyperthyroid rats with clofibric acid. The activity of linoleoyl-CoA chain elongation did not respond to the changes in either the nutritional conditions or the hormonal state of insulin so sensitively as the activity of palmitoyl-CoA chain elongation. The treatment of rats with triiodothyronine caused a marked increase in the activity of linoleoyl-CoA chain elongation; nevertheless, the activity of linoleoyl-CoA chain elongation was not changed by the treatment of rats with clofibric acid. The results suggest that rat liver microsomes contain at least two fatty acid chain elongation systems and that these chain elongation systems are regulated differently by hormones and drugs.