DEFECTIVE GLUCOSE TRANSPORT ACROSS THE BLOOD-BRAIN BARRIER AS A CAUSE OF PERSISTENT HYPOGLYCORRHACHIA, SEIZURES, AND DEVELOPMENTAL DELAY |
| A male infant at the age of 3 months suffered from recurrent seizures. His cerebrospinal fluid (CSF) glucose concentrations were low (0.9-1.9 mmol/L; 16-34 mg/dL), and the ratio of CSF to blood glucose ranged from 0.19 to 0.33; the normal value is 0.65. The potential causes of low CSF glucose concentrations, such as bacterial meningitis, subarachnoid hemorrhage, and hypoglycemia, were not present, and high CSF lactate values would be found in all these conditions except hypoglycemia. In contrast, the CSF lactate concentrations were consistently low (0.3-0.4 mmol/L; 3-4 mg/dL) compared with the normal value (<2.2 mmol/L; <20 mg/dL). These findings suggested a defect in transport of glucose from the blood to the brain. |
| Comment. Assuming that the activity of GLUT-1 glucose transporter in the erythrocyte reflects that of the brain microvessels, a transport assay using his erythrocytes was carried out. The Tmax for uptake of glucose by the patient's erythrocytes was 60% of the mean normal value. A ketogenic diet (a high-fat, low-protein, low-carbohydrate diet) was started, since the brain can use ketone bodies as oxidizable fuel sources, and the entry of ketone bodies into the brain is not dependent on the glucose-transporter system. The patient stopped having seizures within 4 days after beginning the diet. |
| ATP is a high-energy product of metabolism and is often described as the 'energy currency' of the cell. The phosphoanhydride bond of ATP releases free energy when it is hydrolyzed to produce adenosine diphosphate (ADP) and
inorganic phosphate. Such energy is used for synthesis of large and small cellular molecules, cell movement, and uphill transport of molecules against concentration gradients. Primary active transport systems use ATP directly to drive transport; secondary active transport uses an electrochemical gradient of Na+ or H+ ions, or a membrane potential produced by primary active transport processes. Sugars and amino acids are transported into cells by all of these transport systems.
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| The most important primary active transport systems are ion pumps (ion transporting ATPases or pump ATPases)
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| Concentration gradient and electrochemical gradient of ions |
| The permeability of most nonelectrolytes through membranes can be analyzed by assuming that the rate-limiting step is the diffusion within the lipid bilayer. Their permeability across a phospholipid bilayer is experimentally shown to be a function of the partition coefficient into organic solvents. The relative rate of simple diffusion of a molecule across the membrane is therefore proportional to the concentration gradient across the bilayer and to the hydrophobicity of the molecule. For charged molecules and ions, transport across the membrane must be facilitated by a transporter or channel, and is driven by the electrochemical gradient, a combination of the concentration gradient (chemical potential) and the voltage gradient across the membrane (electric potential). These forces may act in the same direction or in opposite directions.In the case of Na+ ions, the concentration difference between outside (145 mM) and inside (12 mM) the cell is about a factor of 10, being maintained by the Na+/K+-ATPase. The Na+/K+-ATPase is an electrogenic, pumping out three Na+ and pumping in two K+ ions, generating an inside-negative membrane potential. K+ leaks out through K+ channels, down its concentration gradient (140 mM-5 mM), further increasing the electric potential. The concentration gradient of Na+ ions and the electric potential power the import and export of other molecules with Na+ against their concentration gradient by symporters and antiporters, respectively. |
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| The pump ATPases are classified into four groups (Table 7.5). Coupling factor ATPases (F-ATPases) in mitochondrial, chloroplast, and bacterial membranes (see Chapter 8) hydrolyze ATP and transport hydrogen ions (H+). This ATPase is also called H+-ATPase (H+-transporting ATPase). In mitochondria, the 'powerhouses' of the cell, the F-ATPase works in the backward direction, synthesizing ATP from ADP and phosphate as protons move down a concentration gradient
generated across the inner membrane by oxidation reactions during metabolism. The proton gradient drives the production of ATP in a process known as oxidative phosphorylation (see Chapter 8). The product, ATP, is released into the mitochondrial matrix, then transported to the outside (the cytoplasmic side) through an ATP-ADP translocase in the mitochondrial inner membrane. This translocase is an example of an antiport system shown in Figure 7.3C); it allows one molecule of ADP to enter only if one molecule of ATP exits simultaneously.
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| MENKES AND WILSON'S DISEASES |
| X-linked Menkes disease is a lethal disorder that occurs in 1 in 100,000 newborn infants and is characterized by abnormal and hypopigmented hair, a characteristic facies, cerebral degeneration, connective tissue and vascular defects, and death by the age of 3 years. A copper-transporting P-ATPase that is expressed in all tissues except liver is defective in this disease (see Table 7.5). In patients with Menkes disease, copper enters the intestinal cells, but is not transported further, resulting in severe copper deficiency. Subcutaneous administration of a copper histidine complex may be an effective treatment if started early. |
| The gene for Wilson's disease also encodes a copper-transporting P-ATPase and is 60% identical with that of the Menkes gene. It is expressed in liver, kidney, and placenta. Wilson's disease occurs in 1 in 35000-100000 newborns and is characterized by failure to incorporate copper into ceruloplasmin in the liver and failure to excrete copper from the liver into bile, resulting in toxic accumulation of copper in the liver and also in the kidney, brain, and cornea. Liver cirrhosis, progressive neurologic damage, or both, occur during childhood to early adulthood. Chelating agents such as penicillamine are used for treatment of patients with this disease. Oral zinc treatment may be useful for decreasing the absorption of dietary copper. |
| Comment. Copper is an essential trace metal and an integral component of many enzymes. However, it is toxic in excess, because it binds to proteins and nucleic acids, enhances the generation of free radicals, and catalyzes oxidation of lipids and proteins in membranes. |
| Cytoplasmic vesicles, such as lysosomes, endosomes, and secretory granules, are acidified by the V-type (vacuolar) H+-ATPase in their membranes. Acidification by this V-ATPase is important for the activity of lysosomal enzymes that have acidic pH optima, and for the accumulation of drugs and neurotransmitters in secretory granules. The V-ATPase also acidifies the extracellular environments of osteoclasts and kidney epithelial cells. Defects in the osteoclast plasma membrane V-ATPase result in osteopetrosis (increased bone density), while mutation of the ATPase in the apical surface of α-intercalated cells of the cortical collecting duct of the distal nephron causes distal renal tubular acidosis. F- and V-type ATPases are struc
turally similar, and seem to be derived from a common ancestor. The ATP-binding catalytic subunit and the subunit forming the H+ pathway are conserved between these ATPases.
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| P-ATPases form phosphorylated intermediates that drive ion translocation: the 'P' refers to phosphorylation. These transporters have an active-site aspartate residue that is reversibly phosphorylated by ATP during the transport process. The P-type Na+/K+-ATPase in various tissues and the Ca2+-ATPase in the sarcoplasmic reticulum have important roles in maintaining cellular ion gradients. Na+/K+-ATPases also create an electrochemical gradient of Na+ that produces the driving force for uptake of nutrients (see below). The discharge of this electrochemical gradient is also fundamental to the process of nerve transmission (see Chapter 40). Mutations of P-ATPase genes cause Brody myopathy (cardiac muscle and fast twitch skeletal muscle sarcoplasmic reticulum Ca2+-ATPase), familial hemiplegic migraine type 2 (α2 type Na+/K+-ATPase), Menkes and Wilson's diseases (Cu2+-ATPases), and familial intrahepatic cholestasis 1 (aminophospholipid flippase).
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Table 7-5.
Primary active transporters in eucaryotic cells. |
| Body_ID: None |
| Various primary active transporters in eukaryotic cells |
| Body_ID: T007005.50 |
| Group | Member | Location | Substrate(s) | Functions |
| Body_ID: T007005.100 |
| F-ATPase (coupling factor) | H+-ATPase | mitochondrial inner membrane | H+ | ATP synthesis,
generation of electrochemical gradient of H+ |
| Body_ID: T007005.150 |
| V-ATPase (vacuolar) | H+-ATPase | cytoplasmic vesicles (lysosome, secretory granules)
plasma membranes (ruffled border of
osteoclast, kidney epithelial cell) | H+ | activation of lysosomal enzymes,
accumulation of neurotransmitters turnover of bone,
acidification of urine |
| Body_ID: T007005.200 |
| P-ATPase (phosphorylation) | Na+/K+-ATPase | plasma membranes (ubiquitous, but abundant in
kidney and heart) | Na+ and K+ | generation of electrochemical gradient of Na+
and K+ |
| Body_ID: T007005.250 |
| | H+/K+-ATPase | stomach (parietal cell in gastric gland) | H+ and K+ | acidification of stomach lumen |
| Body_ID: T007005.300 |
| | Ca2+-ATPase | sarcoplasmic reticulum and endoplasmic
reticulum | Ca2+ | Ca2+ sequestration into sarcoplasmic
(endoplasmic) reticulum |
| Body_ID: T007005.350 |
| | Ca2+-ATPase | plasma membrane | Ca2+ | Ca2+ excretion to outside of the cell |
| Body_ID: T007005.400 |
| | Cu2+-ATPase | plasma membrane and cytoplasmic vesicles | Cu2+ | Cu2+ absorption from intestine and excretion
from liver |
| Body_ID: T007005.450 |
ABC transporter (ATP binding
cassette) | P-glycoprotein | plasma membrane | various drugs | excretion of harmful substances,
multidrug resistance for anticancer drugs |
| Body_ID: T007005.500 |
| | MRP | plasma membrane | glutathione conjugate | detoxification multidrug resistance for
anticancer drugs |
| Body_ID: T007005.550 |
| | CFTR* | plasma membrane | Cl- | cAMP-dependent chloride channel,
regulation of other channels |
| Body_ID: T007005.600 |
| | TAP | endoplasmic reticulum | peptide | presentation of peptides for immune response |
| Body_ID: T007005.650 |
|
| Body_ID: T007005.700 |
*Some ABC transporters function as channels or channel regulators. MRP, multidrug resistance-associated protein; CFTR, cystic fibrosis transmembrane conductance regulator; TAP, transporter associated with antigen processing.
Various examples of primary active transporters (ATP-powered pump ATPases) are listed, together with their location. The F-ATPase is reversible, whereas others catalyze unidirectional ATP hydrolysis reactions.
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| The ATP-binding cassette (ABC) transporters comprise the fourth active transporter family. 'ABC' is the abbreviation for 'ATP-binding cassette', referring to an ATP-binding region in the transporter (Table 7.5). P-glycoprotein ('P' = permeability) and MRP (multidrug resistance-associated protein), which are thought to have a physiological role in excretion of toxic metabolites and xenobiotics, contribute to resistance of cancer cells to chemotherapy. TAP transporters, a class of ABC transporters associated with antigen presentation, are required for initiating the immune response against foreign proteins; they mediate antigen peptide transport from the cytosol into endoplasmic reticulum. Some ABC transporters are present in peroxisomal membrane where they appear to be involved in the transport of peroxisomal enzymes necessary for oxidation of very-long-chain fatty acids. Defects of ABC transporters are associated with various diseases, as shown in the clinical box.
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| Uniport, symport, and antiport are alternate mechanisms of facilitated transport
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| Transport processes may be classified into three general types: uniport (monoport), symport (cotransport), and antiport (countertransport) (see Fig. 7.3). Transport substrates move in the same direction during symport, and in opposite directions during antiport. The movement of one substrate uphill, against its concentration gradient, can be driven by movement of another substrate (usually a cation such as Na+ or H+) down a gradient. Uniport of charged substrates may also be electrophoretically driven by the membrane potential of the cell. The proteins participating in these transport systems are termed uniporters, symporters, and antiporters, respectively (Table 7.3). Some examples are presented below.
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