1. The effect of iodination on the distribution of peptide hormones into the aqueous two-phase dextran–polyethylene glycol system and on the solubility of these hormones in aqueous polyethylene glycol and in water was assessed. Hormones that were studied included insulin, glucagon and parathyroid hormone. 2. The partition coefficient of native insulin in the dextran–polyethylene glycol system showed a minimum (about 1) near the isoelectric point of the hormone (pH 5). Partial iodination of insulin (one atom per molecule) caused little change in the distribution of the hormone. More extensive iodination markedly decreased the partition coefficient in the region of the isoelectric point and displaced the pH value at which the partition coefficient was a minimum towards lower values. 3. The solubility of native insulin in aqueous polyethylene glycol and in water showed a pH-dependence similar to that observed for the distribution in the dextran–polyethylene glycol system. Iodination of insulin decreased the solubility of the hormone in polyethylene glycol and in water in parallel, and decreased the pH value at which solubility was a minimum. The changes in solubility correlated with the degree of iodination and accounted for the changes in distribution observed at high concentrations of insulin. 4. Comparable effects of iodination on distribution and solubility were also observed with glucagon. 5. At concentrations of insulin below its maximum solubility, serum proteins caused a decrease in the partition coefficient of iodinated hormone, but not of native hormone. These effects correlated with the degree of iodination and resulted from a co-precipitation of iodinated insulin with serum proteins.

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