Estimations have been made on abilities of certain tissues of mice injected intravenously with insulin I-131 to concentrate and degrade this compound; change of I-131 from TCA insoluble to TCA soluble form has been used as measure of degradation. Insulin I-131, estimated as TCA insoluble I-131, was concentrated by liver and especially by kidney; formation of TCA soluble I-131 occurred most rapidly in liver. With insulin I-131 held constant but total insulin progressively increased, concentration ratios for TCA insoluble I-131 progressively increased in kidney, and decreased in other tissues. In all tissues, conversion of I-131 from TCA insoluble to TCA soluble form decreased progressively with increase in total insulin. Prior mixing of insulin I-131 with excess guinea pig anti-insulin serum (AIS) resulted in loss of tissue abilities to convert the I-131 into TCA soluble form, and in loss of abilities of all tissues other than liver to concentrate this TCA insoluble I-131.

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