Insulin receptors from human placenta have been labeled by using an oxidative iodination procedure (iodogen-mediated or chloramine-T-mediated), Bolton-Hunter reagent or [3H]acetic anhydride. The oxidative iodination procedure reduces the affinity for 131I-insulin and the receptor protein becomes fragmented into smaller pieces with an S 20,w value of 5–6. However, treatment with Bolton-Hunter reagent or [3H]acetic anhydride does not alter the K d of 131I-insulin binding and the S 20,w value remains unchanged with respect to the native receptor. It is proposed that for labeling multisubunit sulfhydryl-linked protein drastic oxidative iodination procedures should be avoided.
