Erythrocytic systems

Many isoenzymes have been identified from various human tissue sources however, our consideration will deal with six erythrocytic systems that have received routine crime laboratory status. These are phosphoglucomutase (PGM), adenylate kinase (AK), adenosine deaminase (ADA), glucose-6-phosphate dehydrogenase (G-6-PD), 6-phosphogluconate dehydrogenase (6-PGD) and erythrocytic acid phosphatase (EAP). 

Bresler H S era/ 1992 Application of capacitive affinity biosensors HIV antibody and glucose detection Biosensor Design and Application ed P R Mathewson and J W Finley (Washington, DC American Chemical Society) pp 89-104 Keese C R and Giaver I 1994 A biosensor that monitors cell morphology with electrical fields IEEE Eng. Med. Biol. Mag. 13 402-8 Bao J-Z er al 1993 Impedance spectroscopy of human erythrocytes system calibration and nonlinear modelling IEEE Trans. Biomed. Eng. BME-40 364-78... 

Erythrocyte Entrapment of Enzymes. Erythrocytes have been used as carriers for therapeutic enzymes in the treatment of inborn errors (249). Exogenous enzymes encapsulated in erythrocytes may be useful both for dehvery of a given enzyme to the site of its intended function and for the degradation of pathologically elevated, diffusible substances in the plasma. In the use of this approach, it is important to determine that the enzyme is completely internalized without adsorption to the erythrocyte membrane. Since exposed protein on the erythrocyte surface may ehcit an immune response following repeated sensitization with enzyme loaded erythrocytes, an immunologic assessment of each potential system in animal models is required prior to human trials (250). 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

Proteins that can flip phospholipids from one side of a bilayer to the other have also been identified in several tissues (Figure 9.11). Called flippases, these proteins reduce the half-time for phospholipid movement across a membrane from 10 days or more to a few minutes or less. Some of these systems may operate passively, with no required input of energy, but passive transport alone cannot establish or maintain asymmetric transverse lipid distributions. However, rapid phospholipid movement from one monolayer to the other occurs in an ATP-dependent manner in erythrocytes. Energy-dependent lipid flippase activity may be responsible for the creation and maintenance of transverse lipid asymmetries. 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

The processes of electron transport and oxidative phosphorylation are membrane-associated. Bacteria are the simplest life form, and bacterial cells typically consist of a single cellular compartment surrounded by a plasma membrane and a more rigid cell wall. In such a system, the conversion of energy from NADH and [FADHg] to the energy of ATP via electron transport and oxidative phosphorylation is carried out at (and across) the plasma membrane. In eukaryotic cells, electron transport and oxidative phosphorylation are localized in mitochondria, which are also the sites of TCA cycle activity and (as we shall see in Chapter 24) fatty acid oxidation. Mammalian cells contain from 800 to 2500 mitochondria other types of cells may have as few as one or two or as many as half a million mitochondria. Human erythrocytes, whose purpose is simply to transport oxygen to tissues, contain no mitochondria at all. The typical mitochondrion is about 0.5 0.3 microns in diameter and from 0.5 micron to several microns long its overall shape is sensitive to metabolic conditions in the cell. 

The FAD-dependent enzyme glutathione reductase plays a role in the antioxidant system. Glutathione reductase restores reduced glutathione (GSH), the most important antioxidant in erythrocytes, from oxidized glutathione (GSSG) [1, 2]. 

Vitamin B6-coenzyme is involved in a variety of reactions, e.g., in the immune system, gluconeogenesis, erythrocyte fimction, niacin formation, nervous system, lipid metabolism, and in hormone modulation/gene expression [1, 2]. 

Acetylcholinesterase contained in erythrocytes is identical to that found in the nervous system. Its function within erythrocytes may be to control permeability of the cell membrane, to an extent. 

Following exposure of humans to organophosphates, but not specifically methyl parathion, restoration of plasma cholinesterase occurs more rapidly than does restoration of erythrocyte cholinesterase (Grob et al. 1950 Midtling et al. 1985). These findings are supported by studies of methyl parathion in animals. Erythrocyte cholinesterase levels are representative of acetylcholinesterase levels in the nervous system, and, therefore, may be a more accurate biomarker of the neurological effects of chronic low level exposure of humans to methyl parathion (Midtling et al. 1985 NIOSH 1976). 

Janik F, Wolf HU. 1992. The Ca2-i-transport-atpase of human erythrocytes as an in vitro toxicity test system - acute effects of some chlorinated compounds. J Appl Toxicol 12(5) 351-358. 

Figure 13 presents a schematic diagram for drug absorption from the peritoneal cavity. As mentioned above, particles (e.g., erythrocytes, bacteria, colloidal gold, and liposomes) which are not able to pass capillary membranes are removed from the peritoneal cavity via the lymphatic system (Fig. 13, I and II). Relatively low molecular weight compounds (e.g., drugs) are exclusively absorbed via splenic blood capillaries into the portal vein (Fig. 13, III).

Glucose Is Always Required by the Central Nervous System Erythrocytes... 

Many environmental toxins interact with specific cellular receptors, including enzymes, ion channels and ion pumps, and thus provide natural tools for the study of cellular signalling pathways. Palytoxin, a compound isolated from the coelen-terate of genus Palythoa, is one such useful and intriguing compound. The structure of palytoxin was first determined in 1981 independently by Hirata (7) and Moore (2). As one of the most potent marine toxins known, palytoxin has been studied in a variety of systems ranging from erythrocytes to neurons. As a tumor promoter of the non 12-O-tetradecanoylphorbol-13-acetate (TPA) type, palytoxin can also be studied in the context of a growth control system. 

Evidence from a number of systems suggests that ion flux plays a role in palytoxin action. In a wide range of systems, palytoxin effects are accompanied by a change in intracellular cation levels. For example, the influx of Na and/or Ca is associated with palytoxin-stimulated contraction of cardiac and smooth muscle, the release of norepinephrine by rat pheochromocytoma (PC12) cells, and the depolarization of excitable membranes 12—15). Palytoxin also induces K efflux from erythrocytes and thus alters ion flux in a nonexcitable membrane system as well 16-19). In both excitable and nonexcitable membranes, the ultimate action of palytoxin has been shown to be dependent on extracellular cations. The palytoxin-induced effects on smooth muscle and erythroctyes can be inhibited by removing Ca from the media, and the palytoxin-induced release of norephinephrine from PC12 cells can be blocked in Na" free media (ii, 14y 18, 20, 21)... 

Heliantholysin. The major form of heliantholysin is a basic polypeptide chain (pi in the region of 9.8) having a molecular weight of 16,600. Its amino acid sequence has been determined (11). It is powerfully hemolytic for washed erythrocytes derived from a variety of animals, those of the cat being the most sensitive, and those of the guinea pig the most resistant (10). As is true of most hemolytic systems, the biochemical basis for the very large differences in sensitivity of erythrocytes from different animal species is unknown. 

Many microorganisms minimize the effects of the host s defence system against them by mimicking the antigenic stmcture of the host tissne. The eventual immunological response of the host to infection then leads to the autoimmune destmction of itself. Thus, infections with Mycoplasma pneumoniae can lead to production of antibody against normal Group 0 erythrocytes with concomitant haemolytic anaemia. 

Heijn, M., Oude Elferink, R. and Jansen, P. (1992). ATP-dependent multispecific organic anion transport system in rat erythrocyte membrane vesicles. Am. J. Physiol. 262, 104-110. 

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