Thyroid hormone action on mitochondria. 3. Resolution of iodine-containing mitochondria and subfractions by zonal centrifugation.

Mitochondria from the livers of normal rats and L-thyroxine (LTa)-injected rats (5 fig/g, 2 hr before killing) were studied by rate tonal centrifugation to detect early changes in physical properties and in composition. LT, t.reatment causes the fractions containing intact liver mitochondria (as judged by succinoxidase activity) to significantly shift their distribution to the heavier end of a linear sucrose gradient, indicating an increase in sedimentation rate. Iodine distribution follows this shift in mitochondria. Iodine concentration increases 26-fold in mitochondria from LT1-treated rats. In addition, iodine concentrates in the less dense part of the gradient in the LTItreated group. Submitochondrial particles from the livers of injected rats were prepared by 15 min sonication and resolved by isopycnic zonal centrifugation. Distributions of iodine, cytochrome aa3, RNA and monoamine oxidase activity indicate that LT4 is associated with the inner membrane of mitochondria.