Change of rat liver cytochrome oxidase properties by altered thyroid hormones levels

The cytochrome c oxidation capability, the optical properties and subunit content of cytochrome oxidase [EC 1.9.3.1] from livers of hypothyroid, hypothyroid + T3, hyperthyroid and euthyroid rats were tested. Electrophoresis of the enzyme isolated after each treatment revealed a comparable behavior during isolation procedure. Cytochrome oxidase from hypothyroid rats showed a) increase Km and Vmax both in the high and low affinity sites for cytochrome c binding, b) altered optical spectrum, c) increased mitochondrial/cytosolic subunit ratio. These parameters approximated euthyiroid values when the enzyme was isolated from hypothyroid rats which had received a single dose of T3 24 h prior to sacrifice. However, an alteration in optical spectrum was still detected. The enzyme from hyperthyroid rat exhibited a higher affinity for cytochrome c as both Km's decreased. It showed an increase in both mitochondrial and cytosolic subunit content and an excess of heme a+a3 content while its optical properties were similar to control. Thus the thyroid hormone role in modulating the subunit assembly and function of cytochrome oxidase is displayed.