LACTOPEROXIDASE IODINATION OF PIG-KIDNEY NA-ATPASE(, K+)

Lactoperoxidase-catalyzed radioiodination of the tyrosine residues acc essible on the surface of the Na+, K+-ATPase molecule at different iod ide to protein ratios was carried out. The labeled alpha-subunit was i solated by SDS gel electrophoresis and electroblotting followed by ste p-by-step hydrolysis of the COOH-terminal amino acids by carboxypeptid ases B and Y. Radioactivity and quantitative analysis of the protein a nd released amino acids showed that only 3-7 of alpha-subunit tyrosine residues (including the COOH-terminus ones) were accessible to modifi cation. It was found that the LPO-mediated iodination of tyrosine resi dues located on the surface of the Na+, K+-ATPase molecule led to a su bstantial inhibition of its ATP-hydrolyzing activity. Based on these d ata the scheme of the spatial location of the COOH-terminus of the Na, K+-ATPase alpha-subunit was proposed.