The rate of catabolism of fibrinogen is known to be increased by extensive iodination. (MacFarlane, A.S., (1963) J. Clin. Invest. 42, 346-361) Clottability measurements are, however, insensitive to this biological damage. (Regoeczi, E., (1971) J. Nucl. Biol. Med. 15, 37-41). We have used gel permeation chromatography in an attempt to understand the chemical modifications of the protein that underlie this alteration in biological activity. In a comparative study of 125I-labelled fibrinogen labelled by the ICl, chloramine-T, and electrolytic methods, we have found that product 125I-labelled fibrinogen is formed with a molecular weight significantly greater than that of normal fibrinogen. The chloramine-T method gives the highest degree of aggregation and the ICl method gives a product whose molecular weight profile most clearly resembles that of authentic fibrinogen. All of the products are hydrolyzed at similar rates in saline (30 ± 3% in the first 24 h) and in plasma or albumin (2.0 ± 0.5% per day). Some chloramine-T preparations have been noted to be more rapidly hydrolyzed in plasma following a two-day induction period. The rate of breakdown is slower in polyethylene containers than in glass. Our results suggest that ICl preparations of 125I-labelled fibrinogen show the least alteration in chemical properties and may therefore be the best for use in in vivo tracer studies involving radioiodinated fibrinogen.
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