Binding of high density lipoprotein to human fetal adrenal membrane fractions

The properties and regulation of the binding sites for low density lipoprotein (LDL) by ACTH were investigated previously using membrane fractions prepared from human fetal adrenal (HFA) tissue that had been maintained in organ culture. It was found that ACTH caused a 2-fold increase in the number of LDL-binding sites in tissue incubated in vitro. On the other hand, others have demonstrated high affinity binding sites for high density lipoprotein (HDL) in membrane fractions prepared from steroid-producing tissues of the rat. Recently, several investigators suggested that high affinity binding sites for HDL in rat adrenal or ovary might be regulated by ACTH and gonadotropins, respectively. In the present investigation, we evaluated the binding of [¹²⁵I]iodo-HDL to membrane fractions prepared from HFA tissue. The average specific binding capacity for [¹²⁵I]iodo-HDL of membrane fractions prepared from fresh HFA tissue was 316 ng mg protein^-1, and the concentration of [¹²⁵I]iodo-HDL producing half-maximum binding (K_d) was 108 μg ml^-1. Thus, the presence of saturable binding sites for HDL in HFA tissue was confirmed. Nonradioactive HDL competed for binding of [¹²⁵I]iodo-HDL to HFA membranes, whereas LDL was much less effective. Unlike LDL-binding sites, specific binding sites for [¹²⁵I]iodo-HDL were resistant to proteolytic digestion, and [¹²⁵I]iodo-HDL bound to HFA membranes could not be released by glucosaminoglycans. Calcium ions were required for the binding of both [¹²⁵I]iodo-HDL and [¹²⁵I]iodo-LDL to their respective binding sites. When HFA tissue was maintained in organ culture for 2 days in the presence of ACTH, and plasma membrane fractions were subsequently prepared, the number of binding sites for LDL in such membrane fractions was increased 2-fold, but that for HDL was unchanged, compared to the number of binding sites in membranes prepared from tissues incubated in the absence of ACTH. From these results, it is concluded that saturable binding sites for HDL in the membranes of HFA tissue are unlikely to serve an important physiological role in the acquisition of cholesterol for steroid biosynthesis by the HFA.