OESTROGENS are more readily accumulated and retained in responsive cells than in cells that are not their targets1. Cytoplasmic macromolecules2 which specifically interact with oestradiol and other steroid hormones seem to mediate transfer of the agonist to the nuclear chromatin, where the complex is believed to promote expression of the phenotypic effects3–6. It is generally assumed that the hormone diffuses passively to “cytoplasmic” receptors which determine the cellular specificity of response7. But some experiments indicate that steroid hormones interact with components of biological membranes and may enter their respective target cells by a membrane-mediated process8–12 which is saturable and temperature-dependent13–17. We have investigated steroid-binding components associated with the plasma membranes of cells isolated from endometrium, liver and intestinal mucosa. Endometrial and liver cells show substantial binding to oestrogen immobilised by covalent linkage to an inert support, while intestinal cells have no such binding sites. The relative quantity of these steroid receptors at the outer surfaces of cells from diverse tissues corresponds well with the capacity of a given cell to accumulate and retain oestrogen.

Specific binding sites for oestrogen at the outer surfaces of isolated endometrial cells

Cysteine Proteases and Their Inhibitors.

Proteolytic activation of calcium-activated, phospholipid-dependent protein kinase by calcium-dependent neutral protease.

Publisher Summary Studies of receptor binding generally employ highly radioactive ligand derivatives; however, knowledge of the biological activity of the radioactive derivative is essential for a complete interpretation of the binding data. This chapter describes the relation between biological activity and binding, physicochemical detection and assay of receptors, solubilization of membrane receptors, provides examples of membrane-localized receptors, affinity techniques, polyvalent hormone derivatives, and receptor studies, and theoretical mechanisms of hormone–receptor binding and action. The biological response toward many neurotransmitters and hormones, such as insulin, acetylcholine, and epinephrine is rapid both in onset and in the termination of action on removal of these agents; therefore, it may be anticipated that the initial hormone–receptor interaction will also be reversible. The chapter shows that progress has been made toward understanding the way by which hormones, via interactions with cell–surface receptors, modulate overall cellular function. This interaction is only the first in a presumably complicated series of protein–protein interactions within the plane of the cell membrane; the mechanism, whereby the ultimate signal is transmitted to the internal cellular milieu is undoubtedly an equally intricate process. The chapter is considered to be useful in delineating the challenge that the elucidation of these processes presents and in providing a basis and incentive for more discoveries in this field.

Membrane receptors and hormone action.

Isoelectric points and molecular weights of proteins.

Estrogen binding proteins of calf uterus. Molecular and functional characterization of the receptor transforming factor: A Ca2+-activated protease.

Affinity chromatography and the purification of estrogen receptors.

The binding of 3H-noradrenaline to intact cells and microsomes seems not to measure catecholamine receptor interactions, but rather a membrane catechol-binding protein which may be related to the enzyme, catechol-O-methyl transferase. True receptors are probably scarce and detection will require catecholamine derivatives of much higher specific activity than are at present available.

Noradrenaline binding and the search for catecholamine receptors

The adequacy of techniques for purifying oestradiol-binding protein by affinity chromatography has now been established. The eluted protein has been identified as the 4.5S oestradiol receptor on the basis of its heat stability, gel filtration, sedimentation constant and isoelectric focusing.

V. Sica

Papers

Estrogen binding proteins of calf uterus. Molecular and functional characterization of the receptor transforming factor: A Ca2+-activated protease.

Affinity chromatography and the purification of estrogen receptors.

Noradrenaline binding and the search for catecholamine receptors

Purification of oestradiol receptors by affinity chromatography.

Interaction of the estradiol receptor from calf uterus with its nuclear acceptor sites.