Abstract. In the present paper we have examined the properties of the prostaglandin E (PGE) receptor in a transplantable rat Leydig cell tumour (H-540). It appears that PGE1 and PGE2 share a common receptor in membrane particles from this Leydig cell tumour. From saturation analysis and modified Hofstee plots, the specific binding sites for PGE1 can be divided into a high (25%) and low affinity state (75%) with apparent equilibrium constants of dissociation (Kd) of 2.4 · 10−7 mol/l and 4.4 · 10−6 mol/l, respectively. Association rate kinetics at different temperatures employing 5 10−9 mol/l [3H]PGE1 showed that specific binding was time- and temperature-dependent. At 37°C an apparent steady state was achieved after approximately 4 h incubation. The binding of [3H]PGE1 was very tight and no dissociation was observed at 20°C during the first 20 h. The free PGE1 receptor appears to be very unstable. Binding was reduced rapidly by storage at 0°C, by freezing and thawing of membrane particles, and by incubation of concentrated membrane particles. Specificity curves showed that PGA1 and PGA2 displaced [3H]PGE1 from receptor to a somewhat lesser degree than PGE2 and PGE1, whereas PGs of the B, D, I and F series had little or no effect. The fact that inhibition of [3H] PGE1 binding by cold PGE1 occurred in the same concentration range as PGE1 activation of adenylate cyclase, indicates that the specific binding of PGE observed here represents functional receptors coupled to the adenylate cyclase.
Isolation and characterization of plasma membranes containing LH sensitive adenylate cyclase from a Leydig cell tumour