Direct radioiodination

Hadley, S.W, and D.S. Wilbur, Evaluation of iodovinyl antibody conjugates comparison with a p-iodobenzoyl conjugate and direct radioiodination. Bioconjug Chem, 1990. 1(2) 154-61. 

Several applications of photoreactive peptides require the presence of a radionuclide to allow specific and sensitive detection of the photo-cross-linked conjugates. In several cases, radioiodination of tyrosyl moieties and radiolabeled Bolton-Hunter reagents have been used. However, the presence of a radiolabel within the benzophenone photophore is desirable, particularly when the objective is to identify the site of photo-insertion of benzophenone. To this end some radiolabeled, benzophenone-based compounds have been developed and used in peptide synthesis, in particular tritiated Phe(4-Bz) (Scheme 24)J2161 [1-14C-carboxy]-4-benzoylbenzoic acid,1221 and 4-benzoyl-(2,3-3H2)-dihydrocinnamic acid.[154l In addition, 4-(4-hydroxybenzoyl)phenylalanine (Scheme 25) has been directly radioiodinated with Na125I and Chloramine-T)151 ... 

DNNA, or IDNNA without the "I." This intermediate, 2,5-DNNA, underwent direct radioiodination with labeled iodine monochloride in the presence of perchloric acid to give IDNNA with a 40% incorporation... 

Direct radioiodination with Chloramine-T, Chloramine-T solution (50 fily 1 mgmlT ) was added to a mixture of BZM (50 ///, 1 mg ml )y sodium I]iodide (10 fily 37-185 MBq [1-5... 

Direct radioiodination with KIO3. Twenty microliters of 10 mM L-oc-methyltyrosine in 1 N HCl and 5 pi 50 mM KIO3 in water were mixed and transferred to a vial containing 400-1,200 MBq fi IJNal in 0.02 M NaOH. After 10 min, the mixture was purified using an analytical RPjg-HPLC column. The RCY amounted to 80-95%. In order to obtain reproducible high radiochemical yields, 10-pl 0.1 mM Nal solution in water may be added to the radioiodine if... 

An alternative oxidation agent for direct radioiodination is peracetic acid (Kung etal. 1991). 

In case unreacted radioiodide is the only contaminant after, e.g., direct radioiodination, its removal may be accomplished by filtration over AgCl... 

Figure 4. Chemical structures of radioiodinated MPH-Fab and directly radioiodinated Fab fragments.

No significant differences were observed in the radioactivity levels in the blood between the two. However, significant differences were observed in the kidney. MPH-Fab showed high radioactivity levels in the kidney and reached its peak at 1 h postinjection, whereas HML-Fab demonstrated significantly lower levels of the radioactivity in the kidney as early as 10 minutes postinjection. The radioactivity levels in the blood and the kidney after concomitant administration of [ I]HML-Fab and directly radioiodinated Fab ( I-Fab)are also illustrated in Figure 5. 

Radioiodinated derivatives have been prepared to define more closely the target site of a-conotoxins on the acetylcholine receptor (R. Myers, unpublished data). In membrane preparations from Torpedo electroplax, photoactivatable azidosalicylate derivatives of a-conotoxin GIA preferentially label the p and 7 subunits of the acetylcholine receptor. However, when the photoactivatable derivative is cross-linked to detergent solubilized acetylcholine receptor (AChR), only the 7 subunit is labeled. Since snake a-neurotoxins mainly bind to the a subunits of AChR and a-conotoxins compete directly with a-bungarotoxin, the cross-linking results above are both intriguing and problematic. 

The prevailing procedures for direct coupling of 125I to a protein or other molecule are through the use of oxidizing agents. The in situ preparation of an electrophilic radioiodine... 

The following sections discuss the major radioiodination reagents available for direct labeling as well as the main crosslinkers or modification reagents used for indirect labeling techniques. 

Autoradiography offered an alternative technique. The earliest experiments followed the uptake of radioiodine into the thyroid by placing the tissue sections in direct contact with photographic plates (Hamilton, Soley, and Eichom, 1940 Leblond, 1943). Belanger and Leblond introduced the use of liquid emulsion in 1946. Initially this was painted onto sections mounted on microscope slides. Later, slides were dipped into liquid emulsion (Joftes and Warren, 1955) or wrapped around with stripping film (Doniach and Pelc, 1950). Semiquantitative comparisons... 

About one-half of the radioiodine in the targets remains in the caustic-nitrate solution which is stabilized with sodium thiosulfate and stored until the radioiodine decays. During the acid dissolutions, the dissolver off-gas is directed to the iodine sorption system in which the gas is contacted with a 10 hi HNO3—0.4 M Hg(N03)2 solution in an absorption column. Then, the... 

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