The influence of hypothyroidism on the transport of phosphate and on the lipid composition in rat-liver mitochondria.

The influence of hypothyroidism on the transport of phosphate and on the lipid composition in rat-liver mitochondria was examined. It was found that the rate of phosphate transport is reduced (around 40%) in mitochondria from hypothyroid rats compared to that obtained in mitochondria from normal rats. Treatment of hypothyroid rats with thyroid hormone reverses this effect completely. Kinetic analysis of the phosphate transport indicates that only the Vmax of this process is affected, while there is no change in the Km values. The lower rate of phosphate transport in mitochondria from hypothyroid rats is also demonstrated by swelling experiments. There is no significant difference either in the respiratory control ratios or in the ADP/O ratios between these two types of mitochondria. The hepatic mitochondrial lipid composition is altered significantly in hypothyroid rats. The total cholesterol increases, the phospholipids decrease and the cholesterol/phospholipid molar ratio increases (around 40%). Among the phospholipids, cardiolipin shows the greatest alteration (30% decrease in the hypothyroid rats). The phosphatidylethanolamine/phosphatidylcholine ratio also decreases. Alterations were also found in the pattern of fatty acids. These changes in lipid composition may be responsible, at least in part, for the depression of the phosphate carrier activity in mitochondria from hypothyroid rats.