Binding of 6,7- 3H-norethynodrel to the rat uterine cytosol and to human endometrium and myometrium

Abstract

This paper presents a study on the uptake of norethynodrel and its binding to receptor proteins in the rat uterus and the human endometrium and myometrium cytoplasmic fractions. In the rat, 3H-norethynodrel was localized in vivo mainly in the 105,000 × g cytoplasmic fraction of the uterus, vagina, ovary, adrenal, pituitary and the hypothalamus. 3H-norethynodrel in the human and in the rat both in vivo and in vitro binds to uterine cytoplasmic proteins. The sedimentation coefficients of the rat uterine cytoplasmic norethynodrel binding proteins were 7.9S and 4.1S. The approximate molecular weights of the binding proteins were calculated to be 155,000 and 58,000. Norethynodrel binding proteins in the rat and human uterine cytosol were found to be thermolabile in nature. The experiments are suggestive of the presence of sulfhydryl groups in the receptor proteins. In competition studies using human myometrial cytosol, progesterone could displace norethynodrel from its binding sites. It is possible that the binding of norethynodrel to the uterine cytoplasmic proteins may be the primary mechanism of action of norethynodrel at the uterine level.