THYROID-HORMONES ALTER ARRHENIUS KINETICS OF SUCCINATE-2,6-DICHLOROINDOPHENOL REDUCTASE, AND THE LIPID-COMPOSITION AND MEMBRANE FLUIDITY OFRAT-LIVER MITOCHONDRIA

Thyroid-status-dependent changes in lipid/phospholipid profiles in rat liver mitochondria were examined and attempts were made to correlate the observed changes with kinetic parameters of succinate-2,6-dichloro indophenol reductase (SDR) and membrane fluidity. Thyroidectomy caused substantial decrease in the total phospholipid and cholesterol conten ts; the proportion of all phospholipid components was also decreased s ignificantly (50-100% decrease). The energy of activation, E(1), for S DR decreased with a concomitant increase in the membrane fluidity. Tre atment with physiologic doses or replacement doses of L-thyroxine resu lted in partial restoration of lipid/phospholipid components. However, the SDR activity decreased with a simultaneous elevation in the phase -transition temperature and significant decreases in E(1) and E(2). Tr eatment with L-thyroxine restored membrane fluidity to almost euthyroi d values, Regression analysis suggested a role for phosphatidylcholine , phosphatidylethanolanline, phosphatidylserine and phosphatidylinosit ol microdomains in modulating the SDR activity. Membrane fluidity was strongly correlated with total phospholipid, cholesterol, diphosphatid ylglycerol and phosphatidyl-inositol, and the ratio of cholesterol/tot al phospholipid (mol/mol).