Osteoblast-like cells isolated from human bone bioptic specimens were characterized and analysed for the presence of type II estrogen receptor (type II EBS). The amount of type II EBS was measured by a whole-cell assay at 4 °C for 2.5 h using [3H]-estradiol as tracer. Saturation analysis, used to investigate the binding characteristic of type II EBS, resulted in a sigmoid curve. Scatchard analysis showed the binding affinity of the estrogen receptor, yelding a concave plot. The dissociation constant (K(d)), determined from the [3H]-estradiol concentration required for half saturation was about 12 ± 2 nM (SD). The number of type II EBS, estimated at maximum binding, was 197,000 ± 8800 sites per cell. If the regulation of the receptor by flavonoids would be confirmed, the evidence of type II EBS in osteoblast-like cells could suggest a direct action of ipriflavone and others flavonoids on bone density in postmenopausal osteoporosis
