Antigenic and Receptor Sites

Specificity of the M and N blood groups apparently lies in the protein moieties of the cell surface. Wasniowska et al. (1977) have indicated that the major structural difference between the antigens of the M and N blood groups is in the N-terminal amino acids, which are serine and leucine, respectively. In addition, the amino acid presumably at position 5 of each peptide is glycine for the M group and glutamic acid for the N group. 

The first demonstration of intracellular steroid-hormone receptors was by Jensen and Jacobson (1962) for estradiol. Levinson et al. (1972) showed that hormone binding to the glucocorticoid receptor in cultured rat hepatoma cells takes place inside the cell membrane rather than at the cell surface, but did not exclude the existence of other biological actions of steroids mediated by surface membrane receptors. Indeed, Ozegovic et al. (1977) have shown that aldosterone binds to isolated kidney plasma membranes and have proposed that this steroid-membrane interaction may reflect an early event in the transmembrance movement of aldosterone. 

Whether the steroid hormones enter the cell freely, as assumed by some investigators (O Malley, 1971), or by carrier-mediated transport, it is generally accepted that they bind to receptor proteins in the cytoplasm. The steroid-receptor complex apparently undergoes a thermal activation process which allows the complex to enter the nucleus of the cell, where an incompletely understood interaction with chromatin takes place. As a result, specific DNA sequences are transcribed and the new mRNA is released to the cytoplasm, where corresponding translation results in the formation of protein products typical of the target-tissue response to the hormone (Lippman, 1976). This mechanism of hormone action relies almost exclusively for specificity on the presence and number of cytoplasmic hormone receptors. Sheridan (1975) has 

Membrane-bound hormone receptors were detected in the late 1960s. The binding of insulin, glucagon, and epinephrine to isolated plasma membranes of the rat liver or to isolated fat cells and fat cell membranes has been reported (Tomasi et al., 1970 Rodbell et al., 1971 Cuatrecasas, 1971a,b Freychet et al., 1971 Dunnick and Marinetti, 1971). Species-specific interaction between growth hormones and erythrocyte membranes has been shown by Cambiaso et al. (1971). Lef-kowitz et al. (1971) have published a detailed description of the interaction of adrenocorticotropic hormone with its receptors in the adrenal cortex, which appears to be a membrane-associated interaction (Finn et al., 1972). The modes of action for polypeptide hormones and their receptors have been the subject of intense investigation, and a number of reviews on this subject have been published (Cuatrecasas, 1974 Kahn, 1975 Catt and Dufau, 1977). 

It should be remembered, however, that, while adenyl cyclase is found in almost every mammalian tissue examined (excluding erythrocytes), this enzyme can be influenced by a limited number of hormones in any given tissue and often only by one. Thus, although glucagon, epinephrine, and ACTH act on the same adenyl cyclase system of the fat-cell membrane, there appear to be separate receptor sites for each hormone (Bimbaumer and Rodbell, 1969 Bar and Flechter, 1969). 

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