Progesterone-induced secretion of a retinol-binding protein in the pig uterus

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Abstract
A retinol binding protein(s) of MW .apprx. 17,000 was demonstrated in uterine secretions from pigs in the luteal phase of the estrous cycle. This protein was induced in ovariectomized sows treated with progesterone or progesterone plus estradiol, but not in sows given estradiol or corn oil. The vitamin A content of secretions from progesterone-treated animals also increased relative to those in controls. The apparent Kd of the binding protein for retinol was 2.6 .times. 10-6 M. The protein had some affinity for retinoic acid and oleic acid, but did not bind retinyl esters or retinal. The protein probably comprises 5% or less of the total fraction of low MW proteins induced by progesterone. A similar protein was found in allantoic fluid of pregnant animals, suggesting that, like uteroferrin, it serves to transport a H2O-insoluble nutrient from the maternal uterine endometrium to the conceptus.

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