Enzymatic conversion of the major polypeptide chains of thyroglobulin

The subunit composition of fully reduced bovine thyroglobulin consists of two polypeptide chains with molecular weights near 300,000. These two chains have been separated on sodium dodecyl sulfate gels by electrophoresis and were referred to as S and F. The effect of incubating native bovine thyroglobulin with a commercial preparation of horseradish peroxidase resulted in the conversion of S to F (and faster migrating polypeptides). Since this reaction was independent of the presence of H2O2 and iodide, it was necessary to demonstrate that the activity was present in the peroxidase enzyme which could be identified by its heme group. When the enzyme preparation was fractionated on Sephadex G-100, the activity responsible for the conversion of S to F was not associated with the heme peak but with another protein peak which eluted later on the column. Proteolytic inhibitors, such as phenylmethyl sulfonyl fluoride and pancreatic trypsin inhibitor, caused partial inhibition of the activity responsible for the conversion of S to F, whereas inhibitors of peroxidase activity had no effect. Guinea pig thyroglobulin was less susceptible to the proteolytic activity present in the peroxidase preparation than bovine thyroglobulin. The subunit composition of guinea pig thyroglobulin is different from that of bovine, since three bands of different sizes are present. The 300,000 mol wt subunit of guinea pig thyroglobulin (band A) ws largely degraded, whereas the 210,000 mol wt subunit (band B) was partially degraded and the 100,000 mol wt subunit (band C) was unaffected.