Effect of Thyroid State on Characteristics Determining the Susceptibility to Oxidative Stress of Mitochondrial Fractions from Rat Liver
We have studied biochemical and functional characteristics of three mitochondrial fractions resolved by differential centrifugation from the liver of rats in different thyroid states. In particular, the relation between cytochrome and antioxidant content of the mitochondria and their oxygen consumption and response to oxidative stress has been investigated. The cytochrome content is greater in the fractions at higher density, whereas the opposite pattern is shown for antioxidant level. These biochemical characteristics seem to determine functional ones. In fact, the fraction at higher density exhibit greater oxygen consumption and smaller capacity for opposing a oxidative stress. A similar dependence of functional characteristics on biochemical ones was found in altered thyroid states. Hypothyroidism is associated with decreased cytochrome content and oxygen consumption and increased antioxidant level and effectiveness in responding to oxidative stress. The treatment of thyroidectomized rats with triiodothyronine leads to reversion of such a pattern. We are inclined to explain the differences in susceptibility to oxidative stress of the mitochondria by a modulation exerted by thyroid hormone, during the normal maturation process, on their cytochrome content. In effect, such a content is able to determine both respiratory characteristics and sensitivity to pro-oxidants of mitochondria. This sensitivity and the decrease of antioxidants, possibly due to free radical production associated with the increased oxygen flux, make the mitochondria less able to respond to an oxidative challenge.