Chapter 8 : Inactivation and Activation of Biologically Active Molecules

Figure 8.1

Inactivation or activation of biologically active molecules. Reaction of antibody with enzyme or other biologically active molecules may result in loss of biologic function due to stearic hindrance of binding of the activating ligand with its receptor. However, the antigenic sites (epitopes) of an enzyme are usually located on a different part of the molecule from the substrate binding site. Inactivation may occur by alteration of the tertiary structure of the enzyme following reaction with antibody. This reaction affects the structure of the substrate binding site indirectly or inactivates the enzyme molecule. In vivo, either increased or decreased catabolism of an enzyme-antibody complex may occur. The biologic effect of neutralization depends upon the molecule neutralized. Antibodies to cell surface receptors may block or stimulate the receptor or may cause receptor modulation by endocytosis or destruction of the cell by antibody-dependent cell-mediated cytotoxicity (ADCC).

Figure 8.2

Level of abnormality in diabetes. Antibody to insulin may block insulin action, and antibodies to receptors may block or modulate receptors. However, antibody to receptors is not the only way that loss of receptors may occur. Inflammation of islet cells is associated with autoantibody to islet cells and precedes juvenile-onset insulin-dependent diabetes. The islet cell destruction is caused by immune attack by T cells

Figure 8.3

Neuromuscular transmission is mediated by acetylcholine (ACh) released from vesicles in the neuronal axon that bind to acetylcholine receptors (AChR) in the motor end plate. ACh is broken into choline and acetate by acetylcholinesterase, which inactivates ACh. Free choline reenters the neuron by endocytosis. ACh is produced in the vesicle from acetate and choline by choline acetyltransferase. ACh is released from vesicles that fuse with the cell membrane of the motor end plate. Congenital decrease in neuromuscular transmission may be impaired by (1) defects in voltage-gated calcium channels, (2) defects in ACh-containing vesicles, (3) defects in junctional folds, (4) decrease in AChR, and (5) acetylcholinesterase levels in the synaptic cleft. Immune-mediated myasthenia gravis is caused by action of antibodies to AChR to block or cause loss of receptors.

Figure 8.4

The AChR. The subunits of the receptor are arranged around a central ion pore. Each subunit has an intramembranous portion. The scheme shows the unfolded view of the α subunit. The extracellular amino-terminal end of the α subunit contains the acetylcholine binding site (C192 and C193). In autoimmune myasthenia, antibodies may bind to any of the subunits, but most bind to the major immunogenic domain of the α subunit. (From A. I. Levinson and L. M. Wheatley, p. 1354, in R. R. Rich [ed.], Clinical Immunology, Principles and Practice, The C. V. Mosby Co., St. Louis, Mo., 1996.)

Figure 8.5

Comparison of experimental allergic and naturally occurring myasthenia gravis. Antibody to AChRs may be induced in laboratory animals by immunization with AChR from the electric eel or autologous AChR. Autoantibodies to AChR occur spontaneously in humans with myasthenia gravis. Both laboratory animals and affected humans demonstrate progressive muscle weakness, a decrease in AChR, immunoglobulin, and complement deposition, and a mononuclear infiltrate at the neuromuscular junction. On restimulation, the muscle action potential reveals a rapid decline. The thymus of affected humans may contain germinal centers not normally found in the thymus. Since the thymus contains myoepithelial cells with AChR, it is possible that autoantibody production of AChR occurs in the thymus. AcChr, acetylcholine receptor.

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Figure 8.6

Production of myasthenia gravis by anti-idiotypic antibodies to antiagonist antibody. AcChR, acetylcholine receptor.

Figure 8.7

Similarities between hormone receptors, antibodies, and enzymes. (1) Recognition domain (paratope) that distinguishes fine structural differences. (2) Functional domain (constant region). Recognition domains have similar binding properties for ligands and mechanisms of activation of cells through G proteins. The black semicircles represent epitopes on the receptor for antibodies which will not block the binding of the ligand. These sites on antibody molecules are different for each antibody (non-antigen-blocking anti-idiotypes), whereas those on cell surface receptors or enzymes are shared by other receptors or enzymes of the same type.

Figure 8.8

Antibodies to receptors may bind (i) the active site, (ii) epitopes away from the active site, or (iii) epitopes sharing part of the active site. Anti-idiotypes may react with (i) the active site of antibody, (ii) epitopes away from the active site, or (iii) epitopes sharing part of the active site.

Figure 8.9

Relationship of ligand, anti-ligand, anti-idiotypes, receptor, anti-receptor, and anti-idiotypes. An immune response to a ligand may result in antibodies that mimic the receptor and anti-idiotypes that mimic the ligand; an immune response to the receptor may produce antibodies that mimic the ligand and antiidiotypes that mimic the receptor.

Figure 8.10

The ligand-antibody-receptor network. An immune response to a ligand may result in antibodies that mimic the receptor or the ligand (anti-anti-receptor). An immune response to a receptor may produce antibodies that mimic the ligand (anti-antiligand). Antibodies to the ligand may have paratopes that mimic the receptor. In this way, immune response to either ligand or receptor may produce inactivating or activating antibodies. Laboratory animals immunized with insulin develop not only antibodies to insulin but also antibodies to the insulin receptor (anti-anti-insulin).

Figure 8.11

Production of TSH-blocking antibodies by immunization of rabbits with anti-TSH. Anti-idiotype to anti-TSH (anti-anti-TSH) reacts with TSH receptor and blocks TSH binding.

Figure 8.12

Neutralization of tetanus toxin by antitoxin. Antitoxin blocks entry of toxin into neurons. C. tetani releases endotoxin when the organisms die in necrotic tissue. The endotoxin is taken up by axons and delivered to nerve cells, where it inactivates protein synthesis. The loss of activity of inhibitory neurons permits hyperactivity of stimulatory neurons and muscle spasm. Antitoxin provided by active or passive immunization prevents toxin from reaching inhibitory neurons.

Figure 8.13 (A)

Genetic map of HBV. The DNA of HBV is partially doublestranded. The two strands (+ and -) are also called large (-) and small (+) because of the different sizes. The deleted region of the + strand is indicated with a dashed line (-----). The large open reading frame for the HBsAg, the env region, has been cloned, and peptides representing the B- and T-cell epitopes have been identified. (B) Secondary structure of the HBsAg as predicted from the HBV env gene product. The large surface protein contains pre-S1, pre-S2, and S; the middle protein contains pre-S and S; and the small protein contains S only. The pre-S1 region contains the binding region for the hepatocyte receptor for HBV; the S region contains the hydrophobic membrane region inserted into the HBV lipid bilayer. (Modified from A. R. Neurath, B. A. Jameson, and T. Huima, Microbiol. Sci. 4:45–51, 1987.)