Peroxisomal oxidative capacity of brown adipose tissue depends on the thyroid status

The induction of hypothyroidism in rats by methimazole affects interscapular brown adipose tissue (IBAT) mitochondrial and peroxisomal enzyme activities in opposite directions. Hypothyroidism, indeed, decreases both mitochondrial succinate dehydrogenase and beta-oxidation total activities by 35 and 45%, respectively and increases peroxisomal catalase and acyl coenzyme A (acyl CoA) oxidase total activities 3.2- and 1.6-fold, respectively. Administration of a thyroid hormone analogue (3'-isopropyl-3,5-diiodo-L-thyronine) prevents these enzymatic modifications. The effects of hypothyroidism on IBAT mitochondrial enzyme activities seem to be direct, i.e. due to the lack of thyroid hormones, while those on peroxisomal enzyme activities might be indirect, i.e. secondary to the increased thermogenic needs of the rat and mediated by adrenergic stimulation. It is noteworthy that the indirect effects of hypothyroidism on peroxisomes are not observed in liver where acyl CoA oxidase activity is in fact decreased by 40%. In hypothyroid rat IBAT, administration of the peroxisome proliferator nafenopin does not further stimulate the already increased peroxisomal enzyme activities and does not inhibit the already decreased mitochondrial enzyme activities.