Influence of L-thyroxine on kynurenine 3-hydroxylase, monoamine oxidase, and rotenone-insensitive NADH-cytochrome C reductase in mitochondrial outer membrane

Subcutaneous administration of L-thyroxine to rats at a dose of 75 μg/100 g of body weight/day for 12 days resulted in a decrease in the activities of kynurenine 3-hydroxylase, monoamine oxidase, and rotenone-insensitive NADH-cytochrome c reductase in the outer membrane of liver mitochondria to about 50% of control values. L-Thyroxine in concentrations of 1 and 5 μg/ml had no effect on these enzyme activities in vitro. In contrast, certain enzymes localized in the inner mitochondrial membrane showed an increase in activity by the L-thyroxine administration. Membrane-specific responses of intramitochondrial enzymes to thyroid hormone are discussed.