BIOCHEMISTRY - L. Stryer - 1984

VOLUME 3

PART V. MOLECULAR PHYSIOLOGY

CHAPTER 35. HORMONE ACTION

Hormones are chemical messengers that coordinate the activities of various Cells in a multicellular Organism. The term "hormone" was first used in 1904 by William Bayliss and Ernest Starling to describe the action of secretin, a substance secreted by the duodenum that stimulates pancreatic juice secretion. This work gave rise to a highly fruitful concept: 1) hormones are molecules synthesized by specific Tissues (glands); 2) hormones are secreted directly into the bloodstream, which delivers them to their Site of Action; 3) hormones specifically alter The activity of certain sensitive tissues (target Organs or target cells). Hormones are chemically very diverse. Some, such as adrenaline and thyroxine, are small molecules derived from Amino Acids. Others, notably oxytocin, Insulin, and thyrotropin (thyroid-stimulating hormone), are Polypeptides or Proteins. A third group of hormones comprises Steroids, which are derivatives of Cholesterol. The Molecular Mechanism of action has now been elucidated for A number of hormones. It has been established that hormones exert their specific effects through three main pathways: 1) by affecting The rate of synthesis of Enzymes and other proteins; 2) by altering the rate of Enzymatic Catalysis; 3) by changing the permeability of Cell membranes. It is interesting to note that no hormone is an enzyme or a coenzyme. Rather, the Action of Hormones boils down to The regulation of already existing processes.

35.1. Discovery of Cyclic AMP, a Mediator of Many Hormones

A major breakthrough in understanding the Mechanism of hormone Action is associated with the name of Earl Sutherland. The initial goal of his work, begun in the 1950s, was to elucidate the mechanism by which adrenaline and Glucagon promote Glycogen breakdown and glucose formation in the Liver. Sutherland chose this system, first, because these hormones exert a very significant and reproducible effect on Glycogenolysis. Second, this effect develops within a matter of minutes. Third, liver slices are readily obtainable in large quantities. Fourth, the biochemistry of glycogen breakdown was already reasonably well understood (Chap. 16). In fact, Sutherland began his studies on the MECHANISM OF ACTION of adrenaline and glucagon in the laboratory of Carl and Gerty Cori.

At the initial stage of his work, he sought to identify the specific enzymatic step in The conversion of glycogen to glucose that was enhanced by these hormones. To this end, liver slices were incubated in the presence of ³²P_i, and the incorporation of the label into intermediates was determined. It turned out that phosphorylase, rather than phosphoglucomutase or glucose-6-phosphatase, was the rate-limiting enzyme in glycogen breakdown. Moreover, both adrenaline and glucagon increased phosphorylase activity. However, The Mechanism of activation remained unclear. Subsequently, Sutherland discovered an enzyme that catalyzed the inactivation of active phosphorylase. This inactivating enzyme proved to be a phosphatase, suggesting that phosphorylase activation might be due to its phosphorylation. Indeed, when liver slices were incubated with ³²P_i, the rate of label incorporation into phosphorylase was found to increase in the presence of adrenaline and glucagon, and this increase was directly proportional to The Effect of the hormones on glycogen breakdown.

Class="center">Fig. 35.1. Electron micrograph of a somatotrophic cell from the Pituitary Gland. Growth Hormone (somatotropin) is stored in electron-dense granules, which are clearly visible in the micrograph

Thus, these studies established that Phosphorylase is activated by phosphorylation and inactivated by dephosphorylation. This was the first example of ENZYME ACTIVITY REGULATION via covalent modification.

Next, researchers proceeded to analyze hormone-mediated phosphorylase activation in cell-free liver preparations. Strikingly, The addition of adrenaline and glucagon led to phosphorylase activation, just as it did in liver slices. The discovery that hormonal effects could be reproduced in cell-free homogenates marked a new milestone in The Development of biochemistry. Previously, it had not been possible to observe specific hormone actions in cell-free systems, and many biologists believed that hormones could only affect intact target cells. It is interesting to recall how, half a century earlier, the Buchners refuted completely analogous views by demonstrating that Fermentation could proceed in a cell-free Yeast extract. However, unlike glycolytic enzymes, not all Components of the system responsible for the response to adrenaline and glucagon turned out to be soluble. For instance, upon centrifugation of liver cell homogenates, the response to added hormones disappeared. Consequently, an essential part of the hormonal response system was localized in the membrane fraction. Indeed, the hormonal response could be restored by adding the subcellular particle fraction back to the supernatant.

Fig. 35.2. Examples of three chemically distinct classes of hormones: A - adrenaline, an amino acid derivative; B - glucagon, a polypeptide; C - cortisol, a steroid

The Role of subcellular particles was elucidated as follows. When this fraction was incubated with adrenaline and glucagon, a heat-stable factor was produced. The addition of this factor to the supernatant fraction resulted in the activation of phosphorylase. In other words, the hormonal response could be divided into two stages: the interaction of the hormone with the membrane fraction, leading to The formation of a heat-stable factor, and the action of this factor on the supernatant fraction, resulting in phosphorylase activation. The next task was to identify this heat-stable factor, which was obtained in very small amounts. Chemical analysis indicated that it was an adenine ribonucleotide, but with unusual properties. Sutherland described it in a letter to Leon Heppel, whom he contacted in the hope of getting help in elucidating the substance's Structure. At about the same time, Heppel received a letter from David Lipkin describing a new nucleotide obtained by treating ATP with barium hydroxide. Heppel concluded that Lipkin and Sutherland were studying the exact same substance and helped put them in Touch with one another. Indeed, both investigators were studying the same compound, which turned out to be adenosine-3',5'-monophosphate, or, as it is commonly known today, cyclic AMP (cAMP). This chance encounter between the two scientists brought yet another benefit: it immediately made it possible to obtain large quantities of cAMP for biochemical research. Furthermore, the laboratory synthesis of cAMP from ATP and barium hydroxide suggested a plausible pathway for The Biosynthesis of this compound.

Fig. 35.3. Enzymatic Synthesis and degradation of cAMP