Textbook - BIOLOGICAL CHEMISTRY - Gubsky Yu.I. - 2000

Section V. HORMONES IN THE SYSTEM OF INTERCELLULAR INTEGRATION OF BODY FUNCTIONS

CHAPTER 24. HORMONAL REGULATION OF METABOLISM AND CELLULAR BIOLOGICAL FUNCTIONS. II. HORMONES - PEPTIDE AND AMINO ACID DERIVATIVES

24.2. HORMONES OF THE PANCREAS AND GASTROINTESTINAL TRACT

A. Pancreatic Hormones

The Pancreas is a mixed-secretion organ, whose acinar part performs an exocrine function by secreting digestive Enzymes and ions into the duodenum, while The Endocrine Part (islets of Langerhans) produces several peptide hormonal factors. The Synthesis and Secretion of Peptide Hormones are carried out by different Cell types of the islet apparatus:

A (α) Cells — Glucagon;

B (β) cells — Insulin;

D (δ) cells — Somatostatin;

F cells — Pancreatic Polypeptide.

1. Insulin is a polypeptide hormone (M_r 5.7 kD) whose molecule consists of two chains, A and B, containing 21 and 30 amino acid residues, respectively. The peptide chains are interconnected by Disulfide Bonds linking residue A₇ to residue B₇ and residue A₂₀ to residue B₁₉; in addition, a third disulfide bridge connects residues 6 and 11 of the A chain.

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Insulin was the first protein for which the complete Amino Acid Sequence was established (F. Sanger, 1955). Given the great Biomedical Importance of the hormone for the Treatment of Diabetes Mellitus, insulin was also the first protein obtained for pharmaceutical purposes by recombinant DNA biotechnology.

Fig. 24.1. Frederick Sanger (born 1918), an outstanding English biochemist and Professor at Cambridge University. He invented the METHOD FOR DETERMINING the Introduction/19.html">Primary Structure of Proteins and deciphered the primary structure of insulin. A two-time Nobel laureate (1958, 1980).

The Study of the Tertiary Structure of insulin has provided insight into The structure of its active center, which determines the interaction of the protein with the membrane receptor and the realization of hormonal activity (Fig. 24.2).

Fig. 24.2. Spatial arrangement of the A and B chains in the insulin molecule. The dashed line indicates the domain that interacts with the receptor.

Biosynthesis and secretion of insulin

Insulin is synthesized in the Ribosomes of pancreatic B (β) cells as a preprohormone — a protein with M_r 11.5 kD, which, As a result of Limited proteolysis, is successively converted in the Endoplasmic reticulum and Golgi apparatus into a prohormone (M_r 9 kD) and mature insulin.

Insulin molecules are packaged into secretory granules, where they form complexes with zinc ions. The secretion of insulin from The Cell occurs via emiocytosis (exocytosis), which involves the migration of granules to The Plasma Membrane, fusion of the granules with the membrane, dissolution of the membrane, and "extrusion" — the release of the membrane contents into the extracellular space. Insulin secretion is an energy-dependent process, and the main physiological stimulus for secretion is an increase in Blood glucose concentration above the physiological norm (3.3-5.5 mmol/L).

Characteristics of hormonal activity

Insulin is referred to as the "hormone of carbohydrate assimilation and deposition" (V.B. Rosen, 1984). Accordingly, diabetes mellitus (a disease associated with impairments in insulin synthesis, secretion, and/or Insulin Receptor reactivity — Chapter 12) is characterized by persistent hyperglycemia, impaired Carbohydrate METABOLISM, and alterations in lipid and amino acid conversions metabolically linked to glucose metabolism. This hormonal Action of Insulin is due to the following biochemical mechanisms:

(1) Effect on carbohydrate metabolism:

(1.1.) Stimulation of glucose Transport from the extracellular space across the Plasma Membranes into cells — an effect observed predominantly in Muscle cells, adipose tissue adipocytes, and lymphocytes, and is the main cause of the rapid (within a few seconds) decrease in blood glucose levels following an insulin injection. At the same time, insulin does not affect membrane glucose transport in hepatocytes, Brain cells, or Kidney cells; The stimulation of glucose uptake by these Tissues in response to insulin is caused by the hormone's activation of The rate of intracellular glucose utilization (see below);

(1.2.) by promoting glucose utilization in Muscles, the Liver, adipose tissue, etc., via Glycolysis, the Pentose Phosphate Pathway (PPP), and Glycogen synthesis:

(1.2.1.) stimulation of glycolysis in hepatocytes occurs through insulin-mediated activation of the synthesis (induction) of glucokinase—the enzyme that converts glucose to glucose-6-phosphate—as well as Phosphofructokinase and Pyruvate kinase;

(1.2.2.) stimulation of the PPP in glucose metabolism is achieved by activating the glucose-6-phosphate dehydrogenase reaction (this insulin effect is particularly important for adipose tissue and the liver, as it creates metabolic conditions favorable for Lipogenesis);

(1.2.3.) insulin-dependent stimulation of Glycogenesis is driven by the following enzymatic mechanisms:

(a) an increase in glucokinase activity, leading to additional production of glucose-6-phosphate, which can be converted into glucose-1-phosphate, the immediate substrate for glycogenesis;

(b) activation of glycogen synthase: insulin promotes The conversion of the enzyme into its dephosphorylated, active form (this effect is caused by a decrease in cAMP levels under The Influence of insulin, due to the activation of cyclic nucleotide phosphodiesterase and, consequently, a reduction in The activity of the protein kinase that phosphorylates glycogen synthase);

(c) a decrease in Glycogen phosphorylase activity (also resulting from a drop in cAMP concentration and the activity of the corresponding protein kinase); together, these metabolic effects (b, c) of insulin are aimed at storing glycogen in tissues (the anabolic action of the hormone);

(1.3.) inhibition of Gluconeogenesis in the liver;

insulin leads to the inhibition of the synthesis of gluconeogenic enzymes such as PEPCK, fructose-1,6-bisphosphatase, and glucose-6-phosphatase—a process that is slower (compared to The Effect of insulin on glucose transport, glycolysis, and glycogenesis) and requires several hours to manifest.

(2) Effects on Lipid Metabolism.

It is characterized by the stimulation of anabolic pathways of lipid metabolism and increased deposition of neutral fats in cells, which is most pronounced in adipose tissue and the liver. The lipogenic effects of insulin are driven by the following biochemical mechanisms:

(2.1.) activation of higher fatty acid synthesis through an increased influx of corresponding substrates: acetyl-CoA and NADPH, which are produced during glucose metabolism (see above);

(2.2.) activation of triacylglycerol synthesis from Fatty acids and glycerol-3-phosphate, which is also supplied in increased amounts from the glycolytic breakdown of glucose (formed from dihydroxyacetone phosphate via the glycerol-3-phosphate dehydrogenase reaction);

(2.3.) inhibition of lipolysis in adipocytes, driven by a reduction in the cAMP concentration required to activate TG lipase, and by counteracting the lipolytic effects of catecholamines and glucagon.

Accordingly, diabetes mellitus is characterized by the activation of adipose tissue TG lipase, leading to an increased release of non-esterified fatty acids (NEFAs) into Blood Plasma, the stimulation of their conversion into Ketone Bodies (ketogenesis), and, in uncompensated cases, the onset of ketoacidosis. Furthermore, because insulin reduces hepatic production of the main carriers of triacylglycerols and Cholesterol—VLDL and LDL, i.e., "atherogenic Lipoproteins"—type 2 diabetes mellitus (Chapter 16) is associated with an accelerated development of atherosclerosis and obesity.

(3) Effects on Amino Acid and Protein metabolism.

The action of insulin on amino acid and protein metabolism is most pronounced in muscles, the liver, Kidneys, and Connective Tissue; it is anabolic in nature and characterized by:

(3.1.) stimulation of neutral Amino Acid Transport across plasma membranes (an effect most pronounced in muscles);

(3.2.) activation of ribosomal Translation Processes, rRNA synthesis, and certain mRNAs (in muscles, the liver, kidneys, and connective tissue).

(4) Effects on cell growth and proliferation processes.

Insulin exhibits pronounced growth-stimulating effects, which are associated both with the stimulation of the cellular uptake of energetic and plastic substrates for growth (glucose, fatty acids, Amino Acids) and with a direct activating effect on DNA biosynthesis (Replication), accelerating the transition of cells into the S phase. Insulin exerts a positive effect on the proliferation of animal cells in culture, similar to the action of peptide growth factors—epidermal growth factor (EGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF)—whose biological effects are also potentiated by insulin.

MOLECULAR MECHANISMS OF Insulin Action

Despite numerous long-term studies by top research centers worldwide, the molecular foundations of the biochemical and Physiological effects of insulin—specifically the mechanisms by which its receptor-Ligand interaction is translated into specific Responses of the cell's effector systems—remain not fully elucidated.

Receptor Tyrosine Kinases

Insulin receptors located in the membranes of hormone-sensitive cells differ significantly from the protein-peptide Hormone Receptors discussed above, both in their structural Organization and in the mechanisms of hormonal signal Transduction. These insulin receptors share common principles of biochemical organization and function with the receptors of peptide growth factors (EGF, FGF, PDGF) and are termed receptor tyrosine kinases. This class of receptors combines the Functions of both a receptor proper (i.e., a molecular structure that specifically binds the bioregulator), a transducer, and an enzyme, acting as the initiating component of the effector system that executes the cell's biological response to the hormone.

CHARACTERISTICS OF THE structure and functioning mechanisms of receptor tyrosine kinases:

1) receptors are integral proteins that span the plasma membranes; they consist of an outer (extracellular) domain that serves for ligand binding (such as a hormone or growth factor), an intra-membrane segment, and a cytosolic component that performs the catalytic functions of tyrosine kinase (Fig. 24.3);

Fig. 24.3. Model of the transmembrane organization of a receptor tyrosine kinase.

2) in terms of their molecular organization, Insulin receptors are heterodimers composed of Two Types of subunits linked by disulfide bridges to form α₂β₂ oligomers; both subunits are glycosylated. The α-subunit (molecular mass 135 kDa) is located on the outer surface of the cell and contains the insulin-binding sites; the β-subunit (molecular mass 95 kDa) is a transmembrane protein whose cytosolic domain exhibits tyrosine kinase activity (Fig. 24.4);

Fig. 24.4. Scheme of the Molecular organization of receptors (receptor tyrosine kinases) for insulin (a) and epidermal growth factor (b).

3) the binding of a bioregulatory ligand to the extracellular domain leads to the activation of the protein kinase activity of the cytosolic domain of the receptor tyrosine kinase, which then begins to phosphorylate its own tyrosine residues (R-OH) located near the C-terminus of the molecule (the autophosphorylation process):

In turn, the autophosphorylated sites interact with specific target enzyme proteins (E-OH) and phosphorylate them (thereby modifying their catalytic activity), which continues the transmission of the chemical regulatory signal:

An important enzyme phosphorylated in this process is phospholipase C, a key enzyme in stimulating the phosphoinositide cycle, which triggers the activation of protein kinase C and the mobilization of intracellular Ca²⁺ ions. However, this sequence of reactions has been definitively established only for peptide growth factors (such as EGF), while the secondary messengers for insulin and the subsequent enzymatic reactions mediating the multiple cellular effects of this hormone remain not fully elucidated.

Growth Factors and Oncoproteins

Of considerable biomedical interest is the fact that tyrosine kinase activity is possessed by certain oncoproteins—the products of oncogene expression in mammalian cells. Specifically, tyrosine kinases include oncoproteins that are products of the src, erbB, and abl genes, among others, which drive The Development of various types of malignant tumors.

Furthermore, certain oncoproteins are structurally similar (sharing homologous Amino acid sequences) to the Tyrosine Kinase Receptors of BIOREGULATORS: the erbB oncogene (which causes avian erythroblastosis) encodes an oncoprotein with The properties of the EGF receptor, and the sis oncogene (causing simian Sarcoma) is structurally homologous to TGF, and so forth. These data suggest that Human Genome oncogenes (and their corresponding oncoproteins) may also arise as a result of Mutations in genes responsible for the expression of protein (tyrosine kinase) receptors for bioregulators (including insulin) that control the growth and proliferation of normal cells (Yu.I. Gubsky, 1999).

2. Glucagon is a single-chain polypeptide (molecular mass 3.5 kDa) consisting of 29 amino acid residues. The primary site of hormone synthesis is the A (α) Cells of the pancreatic islets, although significant amounts of glucagon can also be produced in other cellular elements of the Cytology/cytology/65.html">Diffuse Endocrine System of the gastrointestinal tract. Glucagon is synthesized as a

prohormone (proglucagon), which is subsequently cleaved into mature glucagon molecules.

The BIOLOGICAL FUNCTIONS OF glucagon involve the REGULATION OF CARBOHYDRATE and lipid metabolism; in terms of its metabolic direction, Glucagon is a counter-regulatory (anti-insulin) hormone, meaning its effects on carbohydrate and fat metabolism are largely opposite to those of insulin. The primary target of glucagon's hormonal action is liver hepatocytes.

(1) The effect of glucagon on carbohydrate metabolism is characterized by:

a) stimulation of Glycogenolysis (without affecting the corresponding process in muscles) through the activation of glycogen phosphorylase; the Molecular Mechanism of the hormone's action involves the activation of membrane adenylate cyclase followed by the engagement of the cAMP-dependent phosphorolytic cascade (Chapter 13);

b) inhibition of glycogenesis by suppressing glycogen synthase activity via its cAMP-dependent phosphorylation;

c) stimulation of glucose synthesis from amino acids; by activating the Synthesis of the enzyme PEP carboxykinase (by increasing the Transcription rate of the PEP carboxykinase Gene), glucagon acts as the most potent activator of hepatic gluconeogenesis.

The sum of these effects (a, b, c) manifests as the hyperglycemic action of glucagon.

(2) The effect of glucagon on lipid metabolism is characterized by its lipolytic action. By increasing cAMP concentration in adipocytes, glucagon activates adipose tissue triglyceride lipase, which leads to the release of non-esterified fatty acids into blood plasma; these free fatty acids serve as energy substrates during β-oxidation and are partially converted into ketone bodies. Under conditions of insulin deficiency, glucagon-dependent acetoacetate production makes a significant contribution to the development of ketonemia observed in diabetes mellitus.

B. Gastrointestinal Hormones

The Cellular Organization of the gastrointestinal tract is characterized by the presence of a diffuse endocrine system comprising up to 16 types of hormone-producing cells. Currently, more than 20 bioregulatory Peptides synthesized by these cells have been isolated and characterized.

In terms of their chemical structure, gastrointestinal hormones (enterohormones) are short-chain peptides and Polypeptides consisting of several to several dozen amino acid residues. Most of the compounds with hormonal and mediator activities belonging to Gastrointestinal Peptides are also synthesized in the Central Nervous system, Hypothalamus, and other Endocrine glands.

The biochemically identified gastrointestinal hormones include: gastrin, cholecystokinin, secretin, gastric inhibitory peptide, vasoactive intestinal peptide, motilin, somatostatin, pancreatic polypeptide, enteroglucagon, enkephalins, substance P, and bombesin (gastrin-releasing peptide).

Gastrin is an enterohormone synthesized by G-cells of the gastric antrum as well as by cells of the duodenal mucosa.

The properties of gastrin are shared by several compounds that have identical 14 C-terminal amino acids but differ in the length of their polypeptide chain (containing 14, 17, or 34 amino acid residues). Each of the molecular forms of gastrin can exist in sulfated or unsulfated form.

The most physiologically active peptide is the antral G-cell peptide, gastrin-17; this hormone stimulates the functional activity of parietal and chief cells of the gastric mucosa, acting as the primary stimulator of gastric Hydrochloric acid and Pepsin secretion.

Hormonally active gastric tumors—gastrinomas—lead to abnormally increased secretion of hydrochloric acid and are accompanied by the development of gastric ulcers that are resistant to standard antacid pharmacotherapy.

Cholecystokinin is a hormone produced by I-cells of the mucosa of the duodenum and proximal jejunum.

Cholecystokinin is a compound characterized by extreme molecular heterogeneity: at least five peptides consisting of 4, 8, 12, 33, and 39 amino acid residues exhibit cholecystokinin activity!

The physiological activity of this enterohormone involves the stimulation of Gallbladder contraction and pancreatic enzyme secretion; cholecystokinin secretion is stimulated by The entry of peptides, amino acids, long-chain fatty acids, calcium, and acid equivalents into the intestine. The most active form is cholecystokinin-8; this peptide has also been detected in the brain, where its unique central effect is the induction of satiety.

Secretin is a hormone secreted by S-cells of the duodenum and proximal jejunum. It is a peptide consisting of 27 amino acid residues.

Secretin stimulates the secretion of bicarbonate and Water by The Pancreas in response to the entry of acidic gastric contents into the intestine.