In a previous report evidence was presented that plasma low density lipoprotein (LDL), but not high density lipoprotein (HDL), is the major source of cholesterol used by the human corpus luteum for progesterone biosynthesis and, that plasma LDL is taken up by corpus luteum tissues via a receptor-mediated endocytotic process. Using membrane fractions prepared from fresh corpus luteum tissue obtained from nine women at various phases of the menstrual cycle, we conducted the present investigation to characterize lipoproteinbinding sites and changes in such binding sites that might occur throughout the ovarian cycle. High affinity, low capacity binding sites for [125I]iodo-LDL and also for [125I]iodo-HDL were detected in membrane fractions prepared from fresh corpus luteum tissue. The interaction of [125I]iodo-LDL with the high affinity binding sites in fresh corpus luteum membrane fractions was prevented by incubation with heparin. Also, the binding capacity of corpus luteum membrane fractions for [125I]iodo-LDL was reduced by treatment with pronase. On the other hand, the specific binding capacity for [125I]iodo-HDL was unaffected by pronase treatment. The binding capacity for [125I]iodo-LDL in membrane fractions prepared from midluteal phase corpus luteum was significantly greater than that of membrane fractions from tissue obtained at any other phase of the cycle, a finding that suggests that changes in progesterone biosynthesis throughout the cycle are positively correlated with changes in the numbers of binding sites for LDL in the corpus luteum.
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