Studies of radioiodinated fibrinogen-II. Lactoperoxidase iodination of fibrinogen and model compounds

In our search for better isotopically labeled fibrinogen for clot detection, we have studied the use of the lactoperoxidase enzyme to catalyze iodination of canine fibrinogen. The optimum conditions which we found require 100 μl of buffer (pH 8·0), 250 μg protein, 5 μg of a crude enzyme preparation, and the desired amount of carrier free radioiodine. The reactants should be equilibrated at 37°C before 10 nmole H₂O₂ is added. The reaction mixture must be incubated with shaking for 30 min to give a maximum yield. Fibrinogen is labeled under these conditions with an efficiency of 60-85 per cent binding. The gel permeation chromatogram showed a normal molecular weight distribution and the product was as stable with respect to in vitro hydrolysis as fibrinogen labeled by the ICl, chloramine-T, or electrolytic methods. The isotopic clottability of the enzymatically labeled product was 83·8 ± 2·5 per cent. Pronase hydrolysis of the labeled fibrinogen gave a yield of 84·3 ± 2·7 per cent iodotyrosine with a small yield of iodohistidine and no iodophenylalanine. Lactoperoxidase does catalyze the iodination of phenylalanine as the free amino acid as well as in a nonapeptide containing two phenylalanyl but no histidyl or tyrosyl residues. The lactoperoxidase/H₂O₂ system shows a greater selectivity for tyrosine labeling than does either chloramine-T or ICl and may therefore become the agent of choice for iodination of proteins requiring long term stability of the protein-iodine bond.