Solubilization and Characterization of a Thyroid Ca 2+ ‐Dependent and NADPH‐Dependent K 3 Fe(CN) 6 Reductase

The thyroid plasma membrane contains a Ca 2+ ‐regulated NADPH‐dependent H 2 O 2 ‐generating system which provides H 2 O 2 for the thyroid‐peroxidase‐catalyzed biosynthesis of thyroid hormones. The molecular nature of the membrane‐associated electron transport chain that generates H 2 O 2 in the thyroid is unknown, but recent observations indicate that a flavoprotein containing a FAD prosthetic group is involved. Solubilization was reinvestigated using 3‐[(3‐cholamidopropyl)dimethylammonio]‐1‐propanesulfonate (Chaps), Triton X‐100, and high salt concentrations. Chaps eliminated about 30% of the proteins, which included a ferricyanide reductase, without affecting the H 2 O 2 ‐generating system. Similarly, Triton X‐100 alone did not extract the NADPH oxidase. An NADPH‐oxidase activity, which was measured in the presence of the artificial electron acceptor potassium ferricyanide, was solubilized by increasing the ionic strength to 2 M KCl. This NADPH‐ferricyanide reductase activity was shown to belong to the H 2 O 2 ‐generating system, although it did not produce H 2 O 2 . It was still Ca 2+ dependent and H 2 O 2 production was restored by decreasing the ionic strength by overnight dialysis. No H 2 O 2 production activity was detected after sucrose density gradient centrifugation of the dialyzed solubilized enzyme, but a well‐defined peak of NADPH oxidation activity with a sedimentation coefficient of 3.71 S was found in the presence of K 3 Fe(CN) 6 . These results suggest that some unknown component(s) (phospholipid or protein) is removed during sucrose density gradient centrifugation. Finally, thyrotropin, which induces NADPH oxidase and regulates H 2 O 2 production in porcine thyrocytes in primary culture, also induced the NADPH‐K 3 Fe(CN) 6 reductase activity associated with the H 2 O 2 ‐generating system. Thus, this enzyme seems to be another marker of thyroid differentiation.