By R. Fadi. Charles R. Drew University of Medicine and Science.
In type II and type III homocystinuria order malegra dxt plus 160 mg overnight delivery, that normally hold the lens of the eye in in which there is a deficiency in the synthesis of methyl cobalamin and of N5- place become frayed and break 160mg malegra dxt plus with amex, causing dis- methyltetrahydrofolate, respectively (both required for the methylation of homo- location of the lens. The skeleton reveals a cysteine to form methionine), serum homocysteine levels are elevated but serum loss of bone ground substance (i. The elongation of the long bones emboli (clots that have broken off and traveled to a distant site in the vascular sys- beyond their normal genetically determined tem) have been reported in almost every major artery and vein as well as in smaller length leads to tall stature. These clots result in infarcts in vital organs such as the liver, the increased concentrations of homocysteine myocardium (heart muscle), the lungs, the kidneys, and many other tissues. Because adenosine and accelerated atherosclerosis), no generally accepted mechanism for these vascu- normally acts as a central nervous system lar events has yet emerged. In addition to a diet low in methionine, very high well as a reduction in cognitive function. The terms hypermethioninemia, homocystinuria (or -emia), and cystathionin- uria (or -emia) designate biochemical abnormalities and are not specific clinical diseases. Each may be caused by more than one specific genetic defect. For exam- ple, at least seven distinct genetic alterations can cause increased excretion of homocystine in the urine. A deficiency of cystathionine -synthase is the most common cause of homocystinuria; more than 600 such proven cases have been studied. Many enzyme deficiency diseases have been discovered that affect the pathways of amino acid metabolism. These deficiency dis- eases have helped researchers to elucidate the pathways in humans, in whom experimental manipulation is, at best, unethical. These spontaneous muta- tions (“experiments” of nature), although devastating to patients, have resulted in an understanding of these diseases that now permit treatment of inborn errors of metabolism that were once considered to be untreatable. Classic PKU is caused by mutations in the gene located on chromosome 12 that encodes the enzyme phenylalanine hydroxylase (PAH). This enzyme normally cat- alyzes the hydroxylation of phenylalanine to tyrosine, the rate-limiting step in the major pathway by which phenylalanine is catabolized. In early experiments, sequence analysis of mutant clones indicated a single base substitution in the gene with a G to A transition at the canonical 5 donor splice site of intron 12 and expression of a truncated unstable protein product. This protein lacked the C-terminal region, a structural change that yielded less than 1% of the normal activity of PAH. Genetic Disorders of Amino Acid Metabolism Amino Acid Degradation Product That Pathway Missing Enzyme Accumulates Disease Symptoms Phenylalanine Phenylalanine hydroxylase Phenylalanine PKU (classical) Mental retardation Dihydropteridine reductase Phenylalanine PKU (non-classical) Mental retardation Homogentisate oxidase Homogentisic acid Alcaptonuria Black urine, arthritis Fumarylacetoacetate Fumarylacetoacetate Tyrosinemia I Liver failure, death Tyrosine hydrolase early Tyrosine Tyrosine Tyrosinemia II Neurologic defects aminotransferase Cystathionase Cystathionine Cystathioninuria Benign Methionine Cystathionine -synthase Homocysteine Homocysteinemia Cardiovascular complications and neurologic problems Glycine Glycine transaminase Glyoxylate Primary oxaluria type I Renal failure due to stone formation Branched-chain amino Branched-chain -keto -Keto acids of the Maple syrup Mental retardation acids (leucine, acid dehydrogenase branched chain urine disease isoleucine, valine) amino acids Since these initial studies, DNA analysis has shown over 100 mutations (mis- sense, nonsense, insertions, and deletions) in the PAH gene, associated with PKU and non-PKU hyperphenylalaninemia. That PKU is a heterogeneous phenotype is supported by studies measuring PAH activity in needle biopsy samples taken from the livers of a large group of patients with varying degrees of hyperphenylalanine- mia. PAH activity varied from below 1% of normal in patients with classic PKU to up to 35% of normal in those with a non-PKU form of hyperphenylalaninemia (such as a defect in BH4 production; see Chapter 48). The genetic diseases affecting amino acid degradation that have been discussed in this chapter are summarized in Table 39. Inborn errors of metabolism in infancy: a guide to diagnosis. The Metabolic and Molecular Bases of Inherited Disease, vol. A statement for healthcare profes- sionals from the nutrition committee of the Council on Nutrition, Physical Activity, and Metabolism of the American Heart Association. The Metabolic and Molecular Bases of Inherited Disease, vol. If an individual has a vitamin B6 deficiency, which of the following amino acids could still be synthesized and be considered nonessential? The degradation of amino acids can be classified into families, which are named after the end product of the degradative path- way. A newborn infant has elevated levels of phenylalanine and phenylpyruvate in her blood. Which of the following enzymes might be deficient in this baby? Pyridoxal phosphate is required for which of the following reaction pathways or individual reactions? A folic acid deficiency would interfere with the synthesis of which of the following amino acids from the indicated precur- sors? Both UDP-glucose and UDP-galactose are used for glycosyltransferase reactions in many systems.
These double-lesioning models are characterized morphologically by neuronal degeneration in the SNpc and ipsilateral striatum purchase malegra dxt plus 160 mg visa. The order of neurotoxic lesioning may inﬂuence the degree of nigral or striatal pathology cheap malegra dxt plus 160 mg with amex. For example, animals receiving 6- OHDA prior to QA exhibit predominantly nigral pathology, while animals receiving QA prior to 6-OHDA show predominantly striatal pathology. This may be due to QA-induced terminal damage or other complex interactions after lesioning that reduce terminal uptake of 6-OHDA. Glial inclusions have not been reported in any of these models indicating a signiﬁcant difference compared with the human condition. Motor deﬁcits in models for MSA and SND are assessed by ipsilateral and contralateral motor tasks (including stepping response, impaired paw reaching, and balance) and drug-induced circling behavior. As described earlier, characteristic drug-induced circling behavior occurs after 6-OHDA lesioning resulting in ipsilateral rotation in response to amphetamine and contralateral rotation in response to apomorphine. The subsequent striatal lesioning with QA diminishes (or has no effect on) amphetamine-induced ipsilateral rotation and reduces (or abolishes) apomorphine-induced contralateral rotation. This observation may be mediated by dopamine release on the intact side (in response to amphetamine) and/or the loss of dopamine receptor activation on the lesioned side (in response to apomorphine). The lack of response to apomorphine has been shown to correlate with the volume of the striatal lesion and is analogous to the diminished efﬁcacy of levodopa therapy observed in the majority of SND patients. A nonhuman primate (Macaca fasicularis) model of SND has been generated through the sequential systemic administration of MPTP and 3- NP (106,109). The parkinsonian features after MPTP lesioning are levodopa responsive, but subsequent administration of 3-NP worsens motor Copyright 2003 by Marcel Dekker, Inc. Levodopa occasionally induces facial dyskinesia as sometimes seen in human MSA. Similar to SND morphological changes include cell loss in the SNpc (typical of MPTP-lesioning) and severe circumscribed degeneration of striatal GABAergic projection neurons (typical of 3-NP lesioning). Despite the similarities with the human condition, the MSA model is characterized by an equal degree of lesioning in the putamen and caudate nucleus, while in human SND the putamen is more affected. In addition, inclusion bodies that may underlie the pathogenesis of SND have not been reported in the nonhuman primate model. The Tauopathies Including Progressive Supranuclear Palsy and Other Tau-Related Disorders The low molecular weight microtubule-associated protein tau has been implicated in a number of neurodegenerative diseases, including Alzheimer’s disease, progressive supranuclear palsy (PSP), Pick’s disease, frontotem- poral dementia with parkinsonism (FTDP), and amyotrophic lateral sclerosis/parkinsonism-dementia complex (ALS/PDC) of Guam. Together these neurodegenerative diseases comprise what is referred to as tauopa- thies, since they share common neuropathological features including abnormal hyperphosphorylation and ﬁlamentous accumulation of aggre- gated tau proteins. Reports in the literature have implicated either alternative RNA splicing (generating different isoforms) or missense mutations as mechanisms underlying many of the tauopathies. Therefore, transgenic mice have been generated that overexpress speciﬁc splice variants or missense mutations of tau (110). One such transgenic line has been developed to overexpress the shortest human tau isoform (111). These mice showed progressive motor weakness, intraneuronal and intra-axonal inclusions (detectable by 1-month postnatal), and reduced axonal transport. Fibrillary tau inclusions developed in the neocortical neurons after 18 months of age implicating age-speciﬁc processes in the pathogenesis of ﬁbrous tau inclusions. An interesting tau transgenic line has been developed in Drosophila melanogaster, where expression of a tau missense mutation showed no evidence of large ﬁlamentous aggregates (neuroﬁbrillary tangles). However, aged ﬂies showed evidence of vacuolization and degeneration of cortical neurons (112). These observations suggest that tau-mediated neurodegeneration is age-dependent and may take place independent of protein aggregation. CONCLUSIONS Our understanding of Parkinson’s disease and related disorders has been advanced through animal models using surgical, pharmacological, and neurotoxicant manipulation. The nonhuman primate, rodent, cat, and pig models have contributed to the development of symptomatic (dopamine modulation), neuroprotective (antioxidants, free-radical scavengers), and restorative (growth factors, transplantation) therapies. In addition, these animal models have furthered our understanding of motor complications (wearing off and dyskinesia), neuronal cell death, and neuroplasticity of the basal ganglia. Future direction in PD research is through the continued development of animal models with altered genes and proteins of interest.
The pressure inside these pul- monary vessels eventually reaches a critical Cora Nari’s rapid heart rate (tachycardia) resulted from a reduced capacity of level above which water from the blood her ischemic buy generic malegra dxt plus 160mg, failing left ventricular muscle to eject a normal amount of blood into moves down a “pressure gradient” from the the arteries leading away from the heart with each contraction buy malegra dxt plus 160mg with visa. The resultant drop capillary lumen into alveolar air spaces of in intraarterial pressure signaled a reflex response in the central nervous system the lung (transudation). The patient experi- that, in turn, caused an increase in heart rate in an attempt to bring the total amount ences shortness of breath as the fluid in the of blood leaving the left ventricle each minute (the cardiac output) back toward a air spaces interferes with oxygen exchange more appropriate level to maintain systemic blood pressure. The hypoxia then stimu- workload of the heart with diuretics and other “load reducers,” attempts to improve lates the respiratory center in the central the force of left ventricular contraction with digitalis and other “inotropes,” and the nervous system, leading to a more rapid administration of oxygen by nasal cannula to reduce the injury caused by lack of respiratory rate in an effort to increase the oxygen content of the blood. As the patient blood flow (ischemia) to the viable heart tissue in the vicinity of the infarction. These sounds represent the bubbling of inspired air as it enters the fluid-filled pul- Active Transport and Cell Death. Most of us cannot remember monary alveolar air spaces. But exactly how cells die from a lack of oxygen is an intriguing question. Pathologists generally describe two histologically distinct types of cell death: Hypoxia necrosis and apoptosis (programmed cell death). Cell death from a lack of O2, such as occurs during a myocardial infarction, can be very rapid, and is considered necro- 2 Decreased mitochondrial sis. The lack of ATP for the active transport of Na and Ca triggers some of the electron transport chain death cascades leading to necrosis (Fig. The influx of Na and loss of the Na gradient across the plasma membrane is Decreased ATP and an early event accompanying ATP depletion during interruption of the O2 supply. For example, the Increased Na+ Increased Ca2+ Na /H exchanger, which normally pumps out H generated from metabolism in exchange for extracellular Na , can no longer function, and intracellular pH may Cellular swelling drop. The increased intracellular H may impair ATP generation from anaerobic glycolysis. As a consequence of increased intracellular ion concentrations, water Mitochondrial Increased plasma permeability enters the cells and hydropic swelling occurs. Swelling is accompanied by the membrane permeability transition release of creatine kinase MB subunits, troponin I, and troponin C into the blood. These enzymes are measured in the blood as indicators of a myocardial infarction Fig. Swelling is an early event and is considered a reversible death. Without an adequate O2 supply, stage of cell injury. Increased ions levels low levels (intracellular Ca concentration is less than 10 M, compared with approximately 10-3 M in extracellular fluid). Fluctuations of Ca2 concentration at can trigger death cascades that involve these low levels regulate myofibrillar contraction, energy metabolism, and other increased permeability of the plasma mem- 2 brane, loss of ion gradients, decreased cytoso- cellular processes. However, when Ca concentration is increased above this nor- 2 2 lic pH, mitochondrial Ca overload, and a mal range, it triggers cell death (necrosis). High Ca concentrations activate a change in mitochondrial permeability called phospholipase that increases membrane permeability, resulting in further loss of ion the mitochondrial permeability transition. They also trigger opening of the mitochondrial solid lines show the first sequence of events; permeability transition pore, which results in loss of mitochondrial function and fur- the dashed lines show how these events feed- ther impairs oxidative phosphorylation. Ca2 - ATPases in the endoplasmic reticulum, and in the sarcoplasmic reticulum of heart and other muscles, sequester Ca2 within the membranes, where it is bound by a low- affinity binding protein. Ca2 is released from the sarcoplasmic reticulum in response to a nerve impulse, which signals contraction, and the increase of Ca2 stimulates both muscle contraction and the oxidation of fuels. Within the heart, another Ca2 transporter protein, the Na /Ca2 exchange transporter, coordinates the efflux of Ca2 in exchange for Na , so that Ca2 is extruded with each contraction. Suggested References Nelson DL, Lehninger AL, Cox MM.
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