Effects of thyroxine on L-cysteine desulfuration in mouse liver

The effect of exogenous thyroxine (T-4) administration on the activity of r hodanese, cystathionase, and 3-mercaptopyruvate sulfurtransferase (MPST) in the mitochondrial and cytosolic fractions of mouse liver was investigated. Three groups of mice were treated for 6 consecutive days with subcutaneous injections of T-4 (50 mu g, 100 mu g, and 250 mu g per 100 g of body wt, r espectively). The other 3 groups were given 100 mu g of T-4 per 100 g of bo dy wt for 1, 2, or 3 days. The dose of 100 mu g T-4 per 100 g of body wt gi ven for 6 days exerted the strongest effect on the activity of all of the i nvestigated enzymes. In comparison to the control, rhodanese activity dimin ished in the mitochondrial fraction by 40% (P < 0.05), cystathionase activi ty diminished in the cytosolic fraction by 15% (P < 0.05), and MPST activit y in the mitochondrial fraction was reduced by 34% (P < 0.05), whereas cyto solic MPST activity was unaltered. Simultaneously, in the liver homogenate, elevated levels of ATP and sulfate were observed after 6 days of T-4 admin istration. Thus, the present results seem to suggest that in the mouse live r, after 6 days of administration of 100 mu g T-4 per 100 g of body wt, the desulfuration metabolism of L-cysteine is diminished, which is probably ac companied by an increase in oxidative L-cysteine metabolism. The dose of 10 0 mu g per 100 g of body wt administered for a shorter period, and the use of a lower dosage (50 mu g T-4 per 100 g of body wt) for 6 days had a stimu latory effect upon MPST activity level, and an increased level of sulfane s ulfur was observed.