To describe quantitatively the in vivo distribution and elimination of insulin, high-performance liquid chromatography (HPLC) separation was applied to the pharmacokinetic study of human insulin labeled with 125I at tyrosine A14 (A14-125I-insulin) as a tracer. Intact A14-125I-insulin levels were determined by HPLC and trichloroacetic acid (TCA) precipitation in plasma and various tissues after its intravenous bolus injection into mice. TCA precipitation consistently overestimated the intactness of A14-125I-insulin compared with HPLC, possibly due to the presence of both a TCA-precipitable intermediate degradation product of labeled insulin found in HPLC elution profiles and reported high-molecular-weight forms of labeled insulin in plasma. Thus, TCA precipitation gave a considerably lower total plasma clearance (Cltot) value than HPLC. The half-life of A14-125I-insulin was prolonged by a simultaneous injection of 8 U/kg unlabeled insulin, and labeled insulin behaved similarly to [14C]inulin (an extracellular fluid marker). The concentration time profiles of HPLC-separated labeled insulin in plasma were analyzed by a noncompartmental moment method, and both Cltot and steady-state apparent volume distribution (VDSS) of A14-125I-insulin were considerably decreased by unlabeled insulin coadministration. In particular, VDSS of labeled insulin decreased by 79%, similar to that of inulin (181 ml/kg), suggesting that the nonspecific binding of labeled insulin to tissues was so small that VD88 of labeled insulin was reduced to the extracellular fluid volume (∼20% of the body weight) when its receptor binding was blocked effectively by unlabeled insulin. This observation, together with the 63% reduction of CI,tot by unlabeled insulin coadministration, demonstrated that saturable, receptor-mediated processes of distribution and elimination are essentially involved in the pharmacokinetics of HPLC-separated A14-125I-insulin.

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