N-methyl oxidation in liver mitochondria of triiodothyronine-treated and thyroidectomized rats

The levels of sarcosine dehydrogenase and acid-nonextractable flavin in the inner matrix of mitochondria of rat liver are decreased in animals treated with triiodothyronine and are elevated in the mitochondria obtained from thyroidectomized animals. Administration of triiodothyronine does not affect the electron-transfer flavoprotein associated with the sarcosine dehydrogenase or the relative amounts of soluble and membrane-bound proteins of the mitochondria. In phosphate-washed mitochondria from either the controls or the triiodothyronine-treated rats, the O₂ uptake equals the total of the ¹⁴C]formaldehyde and β-¹⁴C]serine isolated as reaction products of the sarcosine-¹⁴C]methyl group. In contrast to its restraint of sarcosine or choline oxidation in preparations capable of oxidative phosphorylation, ADP does not inhibit the oxidation of these substrates in mitochondria of rats given triiodothyronine.