Orientation of thyroid peroxidase in hog thyroid microsomes.

The orientation of thyroid peroxidase in hog thyroid microsomes was studied by trypsin treatment, gel filtration, binding to Concanavalin A Sepharose and iodination of thyroglobulin. Trypsin treatment of microsomes did not solubilize the thyroid peroxidase activity completely but solubilized the NADPH-cytochrome c reductase activity almost completely. The apparent molecular size of thyroid peroxidase was not altered by trypsin treatment of microsomes. It was, however, decreased by the same treatment of deoxycholate-treated microsomes. On the other hand, the apparent molecular size of NADPH-cytochrome c reductase was reduced by trypsin without prior deoxycholate treatment. Thyroid peroxidase of microsomes did not bind to Concanavalin A Sepharose. Thyroglobulin added exogenously was not iodinated by microsomes, but endogenous thyroglobulin, which had been associated with microsomes, was iodinated. Similar results were obtained with rough microsomal membranes prepared from crude microsomes by sucrose density gradient centrifugation. These results suggest that thyroid peroxidase is oriented toward the luminal side of the microsomal vesicles.