Effect of thyroidectomy on the kinetics of ADP-ATP translocation in liver mitochondria.

The role of adenine nucleotide translocase (AdNT) in the reduced oxidative metabolism of hypothyroidism has been examined. Both AdNT and respiratory activities in liver mitochondria of thyroidectomized rats were 30% below normal. Mitochondrial AdNT activities were determined by the back-exchange method of Pfaff and Klingenberg (Eur. J. Biochem. 6, 66, 1968). The Km and Vmax of the enzyme were temperature dependent. At physiological temperature, the Km and Vmax of the normal rat AdNT were 10 microM (for external ADP) and 4.73% s-1 (percentage efflux of the labeled adenine nucleotides), respectively. AdNT in hypothyroid rat liver mitochondria exhibited a 25-35% lower Vmax and 75% higher Km when assayed over the temperature range 0 to 37 degrees C. Dixonplot studies indicated that the AdNT in hypothyroidism was two- to threefold more sensitive to atractylate and palmitoyl-CoA inhibitions. In contrast the ADP-ATP translocase in hypothyroidism was more resistant than the control carrier to bongkrekate inhibition. The decrease in the transport of ADP, which is consistent with the decreased oxidative activity associated with hypothyroidism, apparently occurs secondary to changes in the lipid matrix of the inner mitochondrial membrane (F. L. Hoch (1977) Arch. Biochem. Biophys. 178, 535.).