Iodotyrosine deiodinase from bovine thyroid

Abstract

Publisher Summary The iodotyrosine deiodinase represents a mechanism for glandular iodine conservation, the importance of which is evident from the occurrence of goiter and hypothyroidism if the enzyme is genetically deficient or impaired. The enzyme occurs principally in the particulate fraction of the thyroid. The naturally occurring cofactor is nicotinamide adenine dinucleotide phosphate hydrogenase (NADPH). Early studies showed that dithionite could replace NADPH in this reaction. The treatment of the particulate fraction with steapsin and subsequent purification has led to the isolation of a soluble flavoprotein enzyme, essentially homogeneous on gel electrophoresis. However, unlike the native particulate enzyme that can utilize both NADPH and dithionite for the deiodination of the iodotyrosines, the soluble flavoprotein enzyme does not respond to NADPH, but is activated by dithionite and by other strong reducing agents, such as reduced flavins, viologens, and ferredoxins. A crude detergent-solubilized enzyme preparation that is active with NADPH has been prepared from the bovine thyroid particulate fractions. This chapter describes the procedures for the preparation of the purified dithionite-responsive enzyme.