Hormones, structural differences

In addition to numbering the individual carbon atoms, I have labeled the four rings. A, B, C, and D, so that I can easily specify which ring I am talking about later. To anticipate, the structural difference between male and female sex hormones lies almost entirely in the A ring. 

Adrenoceptors interact not only with norepinephrine but also with the adrenal medullary hormone epinephrine and a number of chemically related drugs. However, the responses produced by the drugs in different autonomic structures differ quantitatively or qualitatively from one another. 

Another developmental hormone is the juvenile hormone, which predominates early in larval life later ecdysone predominates. It is a methyl ester of the epoxide of a fatty acid derivative, and there is some evidence that its structure differs in different groups of insects. By a series of coincidences, it was discovered that certain paper toweling prevented the European bug, Pyrrhocoris apterus, from developing into sexually mature adults. Instead, an extra one or two larval molts ensued, and all eventually died without being able to complete metamorphosis. The juvenility factor was traced to particular conifers used in American paper pulp, mainly Abies balsamea, Tsuga canadensis, Taxus brevifolia, and Larix laricina. The active principle was isolated and characterized. It... 

The role of receptor homodimerization is more complicated than simply bringing the cytoplasmic elements of the receptors together. For example, structural studies of EPO and EPO-R indicate that a function of the hormone is to establish a fairly exact receptor alignment, as well as to induce dimerization (Livnah et al, 1998, 1999 Remy et al, 1999 Wilson and JoUiffe, 1999). Based on patterns of cross-hormone and cross-species activities and the known structural differences in the active complexes, exact receptor orientation is probably not as crucial for prolactin and growth hormone systems, although this has not been directly established. 

Steroid hormones bind to and activate receptor molecules that serve as transcription factors to regulate gene expression (Section 31.3.1). These small, relatively similar molecules are able to have greatly differing effects because the slight structural differences among them allow interactions with specific receptor molecules. 

Steroid hormones regulate tissuc-spccinc gene expression. The individual hormones exhibit remarkable tissue selcvlit-ity. even though their structural differences ate relatiu ly minor. Estrogens such as estradiol increa.se uterine cell proliferation. for example, but nut prostate cell prulifcriilion... 

Cholesterol is an important metabolic compound occurring in membranes and lipoproteins. It is also a precursor to bile acids and steroid hormones. The steroid hormones have only small structural differences which cause major differences in functions. This group includes progesterone, testosterone, mineral corticoids, glucorticoids (cortisol), and others. The body has the ability to synthesize and redistribute cholesterol. The main organ that synthesizes cholesterol is the liver. The amount of cholesterol synthesized by the body can be two to three times or more the amount ingested. Cholesterol is not an essential nutrient and can be made in the body from simple compounds via acetyl CoA. 

The abundant IGF-I (somatomedin C), a 70-residue single-chain basic peptide with a sequence and three-dimensional structure homologous to that of proinsu-lin,206.207 jg considered a major mediator of the action of fhe pifuitary growth hormone (GH, somatotropin). Studies in cell culture suggest that GH may induce differentiation of cells, and that IGF-I may then cause a rapid proliferation of the newly differentiated cells. The homologous 67-residue IGF-II may have a similar function in fetal development. TTie cell surface receptor for IGF-I is similar to the insulin receptor, but IGF-II receptor is structurally different. It is a monomeric 250-kDa protein and although it is a substrate for a fyrosine kinase, it has no kinase activity of its... 

The half lives of hormones differ with some having very short half lives of less than a few minutes e.g. ACTH, epinephrine and nor-epinephrine, with other hormones e.g., erythropoietin having half lives of about 2-3 h in the mouse and 8-lOh in the dog. (see Woodman,1997 and Reimers,1999 for detailed descriptions of animal hormone structures and properties). The binding of circulating T3 and T4 to plasma proteins differs, and for the hormones that are bound to plasma proteins, any factors affecting protein synthesis and the ratio of freedotal hormone levels must be considered when interpreting data. 

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