Studies with purified pig thyroglobulin and thyroid enzymes

Abstract

The course of the hydrolysis of pig thyroglobulin by pig thyroid acid proteinase has been followed by estimating the increase and nature of newly exposed N-terminal amino acids. Bonds containing alanine, leucine, tyrosine and phenylalanine have been shown to be most susceptible. Only traces of free amino acids could be detected. By contrast, with [ 131I]thyroglobulin as substrate, the enzyme has been shown to liberate an average of 25–30% of the total iodinated amino acids in the protein during a 24-h incubation period. N- Acetyl- l- phenylalanyl- l- tyrosine hydrolase—also from pig thyroids—has been shown to act on intermediate peptides, formed during the hydrolysis of the labelled protein by the proteinase, releasing further amounts of mono- and di-iodotyrosines. A similar synergistic action occurs, as regards mono-iodotyrosine, when carboxypeptidase is substituted for N- acetyl- l- phenylalanyl- l- tyrosine hydrolase. The possible relevance of these findings to the hydrolysis in vivo of thyroglobulin is discussed.