Functions of cytochrome c in regulation of electron transfer and protein folding.

Cytochrome c, a "mobile electron carrier" of the mitochondrial respiratory chain, also occurs in detectable amounts in the cytosol, and can receive electrons from cytochromes present in endoplasmic reticulum and plasma membranes as well as from superoxide and ascorbate. The pigment was found to dissociate from mitochondrial membranes in liver and kidney when rats were subjected to heat exposure and starvation, respectively. Treating cytochrome c with hydroxylamine gives a partially deaminated product with altered redox properties; decreased stimulation of respiration by deficient mitochondria, increased reduction by superoxide, and complete loss of reducibility by plasma membranes. Mitochondria isolated from brown adipose tissue of cold-exposed rats are found to be sub-saturated with cytochrome c. The ability of cytochrome c to reactivate reduced ribonuclease is now reinterpreted as a molecular chaperone role for the hemoprotein.

Changes in thyroid hormone and insulin status of the brown adipose tissue of cold acclimated rats on short term exposure to heat.

Acclimation of rats to cold caused 45% increase in the concentration of triidothyronine (T3) and 35% increase in the concentration of thyroxine (T4) in serum. Exposure of cold-acclimated rats to heat (12 hr, 37 degrees C) failed to decrease the concentrations of thyroid hormones in circulation. The concentration of T3 in brown adipose tissue (BAT) increased almost 10-fold on cold acclimation. Iodothyronine deiodinase activity also registered 3-fold increase. Exposure of cold-acclimated animals to heat caused decrease in the concentration of T3 in BAT without appreciably affecting T4 concentration. In liver tissue, the changes in hormone concentrations were quite small compared to those in BAT. On thyroidectomy or when fed with propyl thiouracil, rats could not survive exposure to the cold. The concentration of insulin in circulation showed small increase, while that in the tissues showed significant decrease on acclimation of rats to the cold. The concentration of the hormone in BAT registered significant increase on exposure of cold-acclimated animals to heat (12 hr, 37 degrees C). The increase in liver was marginal. The temperature-dependent response of T3 indicates an important role for this hormone in rapid physiological response in BAT.