[Effectors and products of enzymatic diiodotyrosine deiodination by a plasmatic fraction from pig liver].

Effectors and products of enzymatic diiodotyrosine (DIT) deiodination by a cytosolic fraction of pig liver hab been investigated. 13% of the degraded 131I-DIT was found as monoiodotyrosine by thin layer chromatography. The main quantity of the deiodinated DIT was found on the start point of the chromatogram bound to enzyme protein. Tyrosine as a reaction product of enzymatic deiodination of [14C]-IT could not be identified exactly. The liver cytosolic deiodinase is activated by pyruvate; the extent of activation depends on th pyruvate concentration. Diiodohydroxyphenylpyruvate as a product of transamination and theoretically possible intermediate product could be excluded. NADPH 2 and sodium dithionite activated the deiodinase to 1/3, sodium dithionite together with FAD to 1/2 the amount of which was determined for the action of pyruvate. The enzymatic activity in the presence of pyruvate and NADPH2, respectively NADPH2/FAD is identical with the sum of the single activities. The effect of dithionite and sulfite on deiodinase activity depends on the concentration: low effector concentrations increase, while high concentrations decrease the enzyme activity. The liver plasma deiodinase was inactivate quantitatively by reaction with 10(-4) M PCMB; by reaction with 10(-4) M DTNB or NEM the inactivation was 40% only. The inactivation of deiodinase by PCMB was quantitative reversible by cysteine, while inactivation by DTNB was reversible by cysteine to maximal 70% only. Differences between cytosolic and microsomal deiodinases are discussed also in regard to the mechanism of DIT-deiodination by a liver cytosolic fraction with direct participation of SH-groups.