Hormones in mammals

By comparing time-resolved and steady-state fluorescence parameters, Ross et alm> have shown that in oxytocin, a lactation and uterine contraction hormone in mammals, the internal disulfide bridge quenches the fluorescence of the single tyrosine by a static mechanism. The quenching complex was attributed to an interaction between one C — tyrosine rotamer and the disulfide bond. Swadesh et al.(()<>> have studied the dithiothreitol quenching of the six tyrosine residues in ribonuclease A. They carefully examined the steady-state criteria that are useful for distinguishing pure static from pure dynamic quenching by consideration of the Smoluchowski equation(70) for the diffusion-controlled bimolecular rate constant k0,... 

It is well known that the steroid hormones in mammals are biosynthesized from cholesterol. This compound is derived from the acetate-mevalonate pathway through the monoterpene geranyl pyrophosphate, which undergoes several enzymatic reactions to form the triterpene squalene. 

Ecdysone stimulates the synthesis of RNA in tissues. Visual demonstration of the effect is provided by its action on polytene chromosomes of fly larvae (Fig. 26-14).361 Fifteen minutes after the application of ecdysone, a puff is induced on one band of the chromosome a second puff forms at a later time while a preexisting puff diminishes. Thus, like steroid hormones in mammals, ecdysone appears to have a direct controlling effect on transcription. The cuticle-shedding process (ecdysis) is initiated by the brain peptide eclosian. However, the brain may be responding to the ecdysis-triggeiing hormone, a peptide that is secreted by a series of epitracheal glands located in various segments of the body.362... 

Studies of neuromuscular junctions of the autonomic nervous system as early as 1904 led to the suggestion that adrenaline might be released at the nerve endings. Later it was shown that, while adrenaline does serve as a transmitter at neuromuscular junctions in amphibians, it is primarily a hormone in mammals. Nevertheless, it was through this proposal that the concept of chemical communication in synapses was formulated. By 1921, it was shown that acetylcholine is released at nerve endings of the parasympathetic system, and it later became clear the motor nerve endings of the somatic system also release acetylcholine. 

Steroid hormones in mammals are biosynihesixed from cholesterol. which in turn is made in vivo from acetyl-coenrym A (acetyl-CoA) via the mevalonate pathway. Although humans do obtain approximately 300 mg of choleslcnil [ei day in their diets, a greater amount (about I g) is hiiKynlhc-.sized per day. A schematic outline of these biosynthetic paih-ways is shown in Figure 23-. >. 

Which of the following statements about hormones in mammals are correct ... 

The organs believed to be solely responsible for the production of steroid hormones in mammals are the adrenal cortex, the testis, the ovary, and the placenta. This review will assess the histochemical meth-... 

The most important hormone secreted by the adrenal glands for salt balance is aldosterone. In the absence of this hormone, large quantities of sodium are excreted by the kidney and survival is only possible if a 0.9% NaCl solution is available in place of drinking water. Aldosterone, a steroid hormone acts on the kidney to cause sodium resorption in the distal convoluted tubule, partly in exchange for potassium. Increased sodium reabsorption raises the solute concentration of body fluids. Adrenal hormones in mammals also facilitate the resorption of sodium and chloride from sweat and saliva and also cause increased sodium resorption in the gut. Low sodium ion concentration triggers the secretion of aldosterone. But other body conditions like increased potassium concentration, decreased cardiac output and stress too can induce aldosterone secretion. 

Terpenoids are also used as hormones. In mammals, the role of steroids is vital and will be discussed in the article on steroids. The diterpenoid giberellic acid [77-06-5] (14) is a plant growth hormone. 

Some water beetles store in their defensive glands the same sterols that are adrenal hormones in mammals (Figure 7.10). These compounds provide protection against fish, frogs and small mammal predators. Clearly the beetles possess enzymes with which they can degrade cholesterol further and remove most or all of the side chain (Figure 7.10). 

Several years ago, it was discovered that the thyroid gland was also the source of a hypocalcemic hormone having effects in general opposition to those of the parathyroid hormone. This hormone is produced in mammals by the parafollicular C-ceUs and in other vertebrates by the ultimobrachial bodies (45). Originally called thyrocalcitonin, it is now referred to as calcitonin (CT). 

Female sexual development and behaviour in mammals occurs by default and requires no ovarian secretion, and it is only in genetic males that the testis can secrete hormones which destroy this female pattern and superimpose that of the male. Sexual differentiation is not so well defined in fish, and larval exposure to both synthetic estrogens and androgens is widely used in aquaculture to produce monosex cultures. Endocrine disruption of sexual differentiation in fish may therefore reflect both the complexity and diversity of such processes between different species. Some care is required in use of the terms hermaphrodite and sex-reversal since a true hermaphrodite has both functional testes and ovaries and a sex-reversed fish is fully functional as its final sex—both produce the appropriate viable gametes. Such functional sex-reversal is not possible in mammals, but in some species of fish it is the normal developmental pattern. In most of the cases of hermaphroditism or sex-reversal reported in the non-scientific press, there is evidence only for a few ovarian follicles within a functional testis. This may be considered as feminisation or a form of intersex, and is very clearly endocrine disruption, but it is certainly neither sex-reversal nor hermaphroditism. In some cases the terms have even been used to infer induction of a single female characteristic such as production of yolk-protein by males. 

The prolactin receptor, PER, which regulates milk production in mammals, belongs to the same receptor class as the growth hormone receptor. In addition to binding the hormone prolactin, PER also binds and is activated by growth hormone. The extracellular domain of PER forms a very stable 1 1 complex with growth hormone in solution this complex has been crystallized and its structure determined (Figure 13.21). We shall compare this structure with the 1 2 complex of the same hormone with GHR. 

Steroid hormones are crucial signal molecules in mammals. (The details of their physiological effects are described in Chapter 34.) Their biosynthesis begins with the desmolase reaction, which converts cholesterol to preg-... 

---