Type III iodothyronine deiodinase

Evidence has accumulated for the existence of a specific deiodinase for the inner ring of iodothyronines which is further distinguished from the type I enzyme because of its insensitivity to sub-mM PTU concentrations. Thus, type III iodothyronine deiodinase converts T4 to rT3 but not to T3 and produces 3,3 -T2 from T3 but not from rT, (Table I). It has been detected in chick embryo heart [94] and liver [95] cells, monkey hepatocarcinoma cells [96], rat CNS [71,75,97], human [98], rat [98] and guinea pig [99] placenta, and rat skin [100], With higher enzyme activities in cerebral cortex than in cerebellum, the distribution of the type III deiodinase is different from that of the type II enzyme [75], In brain cell cultures type III deiodination appears associated with the presence of glial cells [76,78,79], 

In common with the other deiodinases, the type III enzyme is located in the microsomal fractions of the tissues [97,100] and is activated by thiols. Kinetic analysis 

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Koopdonk-Kool, J. M., van Lopik-Peterse, M. C., Veenboer, G. J. M., Visser, T. J., Schoenmakers, C. H. H., and de Vijider, J. J. M. (1993). Quantification of type III iodothyronine deiodinase activity using thin layer chromatography and phosphor screen autoradiography. Anal. Biochem. 214 329-331. 

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