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Evaluation of murine placental degradation and transfer of [125I]iodo-epidermal growth factor.

Since epidermal growth factor (EGF) has been postulated to play a role in embryonic and fetal growth, a study was undertaken to assess the placental degradation and transfer of maternally administered EGF. Before iodination, mouse EGF was purified to homogeneity by reversed-phase HPLC. Approximately 5 ng [125I]iodo-EGF (approximately 10(6) cpm) were injected iv into day 10, day 13, or day 17 pregnant CD-1 mice; radioactivity in plasma, placentas, and conceptuses was measured up to 2 h after injection. The time course analysis revealed an initial rapid decline in total plasma radioactivity followed by an increase that was maximal by approximately 30 min. Gel filtration (G-15, G-50) chromatography of plasma revealed that by 5 min, radioactivity was associated with free 125I and with material much larger than EGF. No apparent degradation of [125I]iodo-EGF occurred after direct incubation with maternal plasma. Placental radioactivity had an initial phase of decay between 1 and 5 min followed by an increase that became maximal between 30 and 60 min. Extracts of placentas made with 4 M urea in 0.2 M Tris-HCl, pH 8.0, and taken 1-30 min after injection revealed radioactivity coeluting predominantly with [125I]iodo-EGF at 1 min but shifting to mostly free 125I by 30 min. Uptake of radioactivity by conceptuses was not evident until about 15 min, and only free 125I was detected in extracts; the same results were obtained when 5 micrograms unlabeled EGF were injected simultaneously with [125I]iodo-EGF. Incubation of placental mince with [125I]iodo-EGF yielded [125I]MIT as the apparent major radioactive degradation product. Formation of [125I]MIT in vitro was both time- and temperature-dependent. At 37 C, marked formation of [125I]MIT was observed; at 22 C, only a negligible amount was formed after incubation of mince with [125I]iodo-EGF for 60 min. Incubation of [125I]iodo-EGF with kidney mince yielded predominantly free 125I. When tissues were incubated directly with [125I]MIT, kidney tissue deiodinated the [125I]MIT, but placental tissue did not. When [125I]iodo-EGF was incubated for up to 2 h with kidney tissue in the presence of excess MIT (unlabeled), the major degradation product eluted as [125I]MIT, instead of free 125I. These findings suggest that the mouse placenta can readily bind and then degrade [125I]iodo-EGF to constituent amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)