Glycogen Storage Disease Type IV - Essay Example

A female patient, 11 weeks old, presents the following symptoms; an enlarged liver and spleen and failure to thrive. You order a liver biopsy that indicates she exhibits early stages of liver cirrhosis and histological studies (see image below) show a periodic acid-Schiff (PAS)- positive (indicates presence of glycogen), amylase-resistant, coarsely clumped material in the liver cells that is consistent with abnormal and insoluble glycogen. Explain the most likely metabolic basis for the child's symptoms and identify the metabolic disorder she presents. The above mentioned symptoms correlate with the disorder 'Glycogen storage disease type IV', also known as "Andersen's disease". It is a rare disease that leads to early death. Glucose, a source of energy, is stored in the liver and muscle in the form of its polymer, glycogen. The conversion of glucose to glycogen, a process of energy storage, is achieved by a number of enzymes, the absence or deficiency of which leads to the GSDs resulting in inborn errors of glycogen metabolism. These disorders arise due to deficiency of enzymes involved in the glyconeogenesis or due to abnormal functioning of the glycogen breakdown enzymes. GSDs are categorized based on the type enzyme deficiency and the tissue affected. The systems involved in glycogen synthesis disorders are the liver and muscle, which are the primary sites of energy storage. GSD Type IV, also known as amylopectinosis, is usually fatal and leads to death by age 4. Some of the clinical symptoms include hepatomegaly, failure to thrive, splenomegaly, cirrhosis and lumbar lordosis. Type IV disorder arises due to deficiency of the glycogen branching enzyme amylo-1,4-1,6 transglucosidase, which leads to the formation of abnormally structured glycogen, having amylase molecules with low solubility, leading to glycogen precipitation in the liver, the heart and other tissues. The condition ultimately leads to early death (Ozen, 2541-53). Type I diabetic patients who are erroneously prescribed too high of an insulin dose in their insulin regimen can present liver disease symptoms similar to those presented in glycogen storage diseases. Explain the biochemical basis for this finding. Insulin regulates the amount of glucose absorbed or released from the cell. The glucose absorbed from the blood is stored in the form of glycogen in the liver. One of the important functions of insulin is glycogen synthesis, brought about by activating the enzyme hexokinase. This in turn, phosphorylates glucose, thus, trapping it within the cell, in the form of energy. Insulin also inhibit the activity of glucose-6-phosphatase and activates phosphofructokinase and glycogen synthase, thus helping the hepatocytes to havest excess glucose in the form of glycogen. But, in the absense of insulin, as in case of type 1 diabetes, glucose circulation in the blood increases and the cells fail to harvest it for energy. In the absence of energy the cells activate the enzymes involved in the breakdown of glycogen (glycogenolysis).Under the conditions of cell starvation triggered by lack of insulin, glucagon, a counter regulatory hormone,is activated, which, again stimulates the process of glycogenolysis in hepatocytes, thus releasing energy. When insulin is administered from an outside source, in excess, (eg: injections), insulin activates the formation of glycogen from glucose and inhibition glycogenolysis. Therefore, in presence of excess insulin, there is a surplus production of glycogen, similar to that of glycogen storage disorders. Also, there is inhibition of glucose-6-phosphate, similar to deficiency of the same as in glycogen storage disorders (Bowen). 3.Helicobacter pylori is a bacterium that colonizes the upper gastrointestinal tract in humans and is the causative agent of chronic gastritis, ulcers, and possibly gastric cancer. Knowledge of the intermediary metabolism of this organism would be helpful in developing effective drug therapies to treat these diseases. The citric acid "cycle" in H. pylori is a noncylic branched pathway that is used to produce biosynthetic intermediates instead of metabolic energy. Succinate is produced in the "reductive" branch, whereas -ketoglutarate is produced in the "oxidative" branch. The two branches are linked by -ketoglutarate oxidase reaction. The pathway is shown in the diagram to the right and information on the enzymes of this pathway is given in the table below; a)Compare and contrast the citric acid cycle in H. pylori with the citric acid cycle in mammals. H. pylori Has a Branched Incomplete Cytric acid cycle (Kelly, 2001). 1. It lacks some of the enzymes of the pathway, such as pyruvate dehydrogenase multi-enzyme complex, used in the Oxydative decarboxylation of pyruvate and instead this reaction is catalyzed by an unusual four-subunit pyruvate:flavodoxin oxidoreductase (POR) enzyme. 2. Has a single copy of aconitase, instead of the two subunits present in mammals and E.coli. 3. Isocitrate dehydrogenase acts as a critical branch point between the CAC reactions and the glyoxylate bypass during growth on C2 compounds like acetate. H. pylori probably lacks a functional glyoxylate bypass. 4.The next step in the oxidative branch of the conventional CAC is the oxidative decarboxylation of 2-oxoglutarate in the presence of CoA to generate succinyl-CoA with the release of CO2. This reaction is commonly catalyzed by the 2-oxoglutarate dehydrogenase multi-enzyme complex. H. pylori lacks this enzyme complex. Instead, oxidation of 2-oxoglutarate is carried out by an oxygen-labile 2-oxoglutarate:acceptor oxidoreductase enzyme which shares a number of biochemical and structural features in common with the POR enzyme. 5. SCS catalyzes the sole reaction of the CAC in which a nucleotide triphosphate is generated from the conversion of succinyl-CoA to succinate and CoA. The enzyme can also operate in the reverse direction to generate succinyl-CoA. There is no biochemical evidence for the presence of SCS activity in H. pylori. The absence of this enzyme would be consistent with other evidence that the CAC in H. pylori consists of a reductive branch ending in succinate and an oxidative branch ending in succinyl-CoA, with no physiological need to connect the two branches. 6. The interconversion of fumarate and succinate can be carried out by two enzymes: succinate dehydrogenase, which is expressed under aerobic conditions, or fumarate reductase, which is induced under anaerobiosis. H. pylori lacks succinate dehydrogenase, but has a fumarate reductase. It also has a fumarase which strongly favours fumarate formation. 7. H.pylori lacks allosteric inhibition of citrate synthase by NADH, supporting a role in biosynthesis rather than energy conservation. 8. H. pylori shows the presence of NAD-linked MDH activity and also malate synthase without obvious genes encoding them. 9. H.pylori shows the apparent absence of anaplerotic reactions for oxaloacetate synthesis. The function of the C6-branch CAC reactions in H. pylori is clearly biased toward biosynthesis of 2-oxoglutarate and succinyl-CoA, the key enzyme being OOR, whereas the C4 branch is concerned with fumarate respiration to allow redox balancing and energy conservation. The key enzyme here is fumarate reductase. b)What enzymes might serve to regulate the citric acid cycle in H. pylori Citrate synthase, aconitase, isocitrate dehydrogenase, alpha-ketoglutarate oxidase, fumarate reductase, fumarase, malate dehydrogenase,malate synthase (Piston, 1999). Also, the malate: quinone oxidoreductase, which should not really be considered part of the CAC but as a mechanism by which electrons from malate can be used for respiration. H. pylori was found to metabolize aspartate rapidly, and the initial product of aspartate metabolism was identified as fumarae, indicating a role for aspartase in fumarate formation (Kelly, 2001). c)What enzymes of the citric acid cycle in H. pylori might be used as drug targets for persons suffering from ulcers, gastritis, or gastric cancer Alpha-keto glutarate (Tsugawa, 2008); Fumarate reductase (Zhongming Ge, 2002) 4. In an experiment studying the activity of urea cycle enzymes, cultured hepatocytes were treated with an alpha-toxin that served to insert pores in the plasma membrane and allowed entry of molecules from the culture media. These hepatocytes were incubated in a physiological medium containing 14CO2, aspartate, and ammonium chloride in addition to other essential factors (trial 1). The 14C label appeared in urea as expected. In trial 2, the addition of 14CO2 was followed a few minutes later by the addition of an excess of unlabelled arginine (lacks 14C) and no decrease in the specific activity of urea (ratio of 14C incorporated in urea) was observed. Trial 3 was the same as trial 2 except that the addition of 14CO2 was followed a few minutes later by the addition of an excess of unlabelled citrulline (lacks 14C). A decrease in the specific activity of urea (ratio of 14C incorporated in urea) was observed in trial 3. a)What do these results suggest about the substrate channeling between the different urea cycle enzymes exploited in the study The permeabilized cells synthesized [14C]urea using succinate as a respiratory substrate; Trial 2 indicates very tight channeling of arginine between argininosuccinate lyase and arginase was shown by the fact that the addition of excess of unlabeled arginine to the incubations did not decrease the percentage of counts found in urea or increase that found in arginine. Trial 3 shows Channeling of citrulline from its site of synthesis, although not as tightly as arginine. The product formed on addition of citrullin to the arginine medium in trial 2, leads to the formation of argininosuccinate. The channeling of argininosuccinate between its synthetase and lyase was demonstrated by similar observations; unlabeled argininosuccinate added in excess decreased the percentage of counts in urea. Channeling of citrulline from its site of synthesis by ornithine transcarbamylase in the mitochondrial matrix to argininosuccinate synthetase in the cytoplasmic space was also shown (Chueng, 1989). b) What does this suggest about the physical organization of these enzymes Is this consistent with what we discussed in class about the organization of the urea cycle The cytoplasmic enzymes of the urea cycle are considered to be soluble proteins. The results presented here show that these enzymes are not randomly distributed but are organized in such a way that efficient transfer of the metabolic intermediates can occur between them. The channeling of arginine from argininosuccinate lyase to arginase is extremely tight, while that of argininosuccinate between the synthetase and the lyase is apparently less tight but still readily demonstrable. Another level of organization is represented by the channeling of citrulline from its site of synthesis by ornithine transcarbamylase in the mitochondrial matrix, across the mitochondrial membranes, to the active site of argininosuccinate thetase in the cytoplasm. Clearly, the three cytoplasmic enzymes must be organized about the mitochondria (Chueng, 1989). 5. A newborn infant with potentially fatal hyperammonemia (excess ammonia in the blood) was found to have no N-acetylglutamate synthase activity in the liver. Administering N-carbamoylglutamate restored the ammonia levels to normal. a) How does the synthase deficiency affect an increase the level of ammonia N -acetylglutamate synthetase (NAGS), a mitochondrial enzyme, catalyses the production of N-acetylglutamate from Acetyl CoA. N -acetylglutamate (NAG), produced by NAGS, activates carbamyl phosphate synthetase (CPS), another mitochondrial enzyme, which catalyzes the first reaction of urea cycle, wherein, carbomyl phosphate is synthesized. Thus, NAGS is essential for the urea cycle. In people with N-acetylglutamate synthase deficiency, N-acetylglutamate is not produced, thereby inactivating the carbomyl phosphate synthase, which in turn fails to initiate the urea cycle. Failure to initiate the urea cycle leads to accumulation of excess nitrogen in the blood in the form of ammonia (Roth, 2008; Caldovic, 2006). b) Would prescribing a protein-free diet for the infant, instead of administering N-carbamoyl-glutamate, alleviate the hyperammonemia Why or why not A high protein rich diet is not advised as, when on high-protein diets, energy is produced in the form of glycogen by the oxidation of the carbon skeletons of amino, but the amino nitrogen is left out to be excreted. And as the person already has a deficiency of enzymes required to regulate the activity of the urea cycle, the excess nitrogen on the aminoacid is accumulated in the form of ammonia. 6.As a pediatric gastroenterology resident, you see two patients that seem to present the same symptoms; The first is a white male infant, the third child born to a 30-year-old mother and a 32-year old father. Both parents are of Mennonite origin from the Province of Quebec, Canada. The mother had 2 maternal aunts who died in the "first year" of life from unknown causes. Two of her paternal aunts also died in the first month of life. The father has 7 brothers and 3 sisters; all of them are living and well. He has a maternal aunt who died at 7 months of age from unknown causes. The maternal parents on the father's side and the paternal parents on the mother's side are related (the exact relationship is not known). The first child of this couple also suffered from an identical disease and died at the age of 9 days. The second child is 51/2 years old with normal growth, development and intelligence. This patient displayed poor feeding from 5th day and completely stopped feeding on the 7th day. On the 9th day he was irritable and had arching of his back and rigidity on his limbs. His chest and skull x-ray were normal. You order several laboratory tests which give the following results: lumbar puncture showed clear C. S. F., containing 4 WBC per high power field, no RBC, 45 mg% of protein, 46 mg% of sugar, and no bacteria detected in gram stain smears. CSF culture was negative. Blood glucose was 83 mg%. Urine contained no bile, protein or sugar, but had a "sweet smell" and a strong reaction with the 2,4-dinitrophenylhydrazine test for keto acids. The second is a 14 month old female Infant who was the fourth child of parents who were unrelated. Her three siblings, all male, and both parents, were normal. During the first year of life the girl had several immunizations and suffered mild illnesses without becoming acutely ill. She consumed large amounts of milk, eggs, and meats during infancy. Except for the increased body weight, the rest of physical examination was unremarkable. Her physical growth normal but her development was slowed overall. You order several laboratory tests which give the following results: lumbar puncture showed clear C. S. F., containing 4 WBC per high power field, no RBC. CSF culture was negative. Blood glucose was 80 mg%. Urine contained no bile, protein or sugar, but also had a "sweet smell" and a strong reaction with the 2,4-dinitrophenylhydrazine test for keto acids. You treat both patients by implementing a dietary thiamine supplement. Patient 2 responds to the thiamine treatment and is asymptomatic within 3 months. Patient 1 does not respond and you modify the treatment of patient 1 to incorporate a diet free of leucine, isoleucine and valine to be given by naso-gastric tube. At the age of 2 months, the patient weighed 4.6 kg, and was completely free of symptoms. a)Explain the biochemical basis of your diagnosis and treatments of each patients' condition What is the significance of Mennonite origin of the parents of patient 1 The 'sweet smell' of the urine and the subsequent reaction with 2,4-dinitrophenyl hydrazine, a test specific for aldehydes and ketones, lead to the identification of the condition as 'Maple syrup urine disease'( MSUD). MSUD is a caused due to a deficiency in the 'branched chain alpha-ketoacid dehydrogenase' (BCKDH) enzyme, which catalyzes the decarboxylation of the alpha keto acids of branched chain amino acids (BCAA), like valine, leucine, and isoleucine, to the branched chain acyl-CoAs. These are in turn converted to acetyl-CoA, succinyl-CoA and acetoacetate. . In case of deficiency of BCKDH, the oxidative decarboxylation reaction is blocked, thus resulting in the accumulation of the BCAAs, their alpha-ketoacid forms and alpha-hydroxy forms, which appear in the urine, thus giving a positive reaction with 2,4-dinitrophenylhydrazine (Festschrift, 1998; Fingerhut, 2008). The late age of onset, with near normal development, and the appearance of symptoms after the immunization, and mild symptoms even after consumption of a protein rich diet of egg and meat during her first year, represents a case of 'intermediate MSUD' or the 'thyamine responsive MSUD' in patient 2. Hence only a thyamine supplement was provided, without actually restricting BCCAs in diet. And her positive response to thyamine, an essential enzyme in carbohydrate and amino acid metabolism, confirms the diagnosis. Whereas patient 1 represents a 'classic case of MSUD'/ thyamine non-responsive, and hence required a more stringent treatment, with a diet free totally free of BCCAs. MSUD is a metabolic autosomally inherited recessive disorder, resulting due to founder effect, with a high prevalence in Mennonite children. Previous studies show the carrier frequency of MSUD in the Old Order Mennonites is to be 7.96% with a mutation allele frequency of 4.15%, which has been shown to have subsequently increased due to repeated cycles of sampling effects, population bottlenecks, and genetic drifts. Microsatellite studies show a decrease in genetic diversity of Mennonite populations. Hence, they play a significant role in passing on the defective BCKDH to their progeny at a greater frequency than other ethnic groups (Puffenberger, 2003). 7. You have diagnosed one of your patients with severe combined immunodeficiency syndrome (SCIDS). SCIDS is characterized by a lack of an immune response to infectious diseases. In one variant of SCIDS, patients have a deficiency of adenine deaminase, elevated dATP levels, and lower than normal levels of other deoxynucleotides. Explain how an adenine deaminase deficiency affects the levels of the deoxynucleotides. ADA catalyzes the deamination of adenosine and deoxyadenosine in the purine salvage pathway. The absence of the enzyme results in SCID. The effects on the immune system result almost exclusively from the build-up of one of the substrates, deoxyadenosine (dAdo), derived from the rapid DNA turnover occurring in lymphoid cells. When the enzyme is defective, the dAdo is metabolized to the deoxyribonucleotide dATP (i.e deoxyadenosine is phosphorylated to dATP), which accumulates in erythrocytes, thymic cells and B cells, one of the mechanisms of lymphotoxicity involving inhibition of DNA synthesis (Fairbanks, 1994; Qasim, 2004). 8.The compound trimethoprim (shown below) is prescribed as a bacteriostatic antibiotic. It decreases the availability of thymidylate in some bacteria. Explain the basis of trimethoprim's action. Trimethoprim inhibits the synthesis of tetrahydrofolic acid by inhibiting the bacterial dihydrofolate reductase enzyme. Bacteria cannot take up the environmental folic acid and therefore, have to synthesize their own tetrahydrofolic acid. Absence of dihydrofolate reductase interferes with bacterial DNA synthesis, as in the absence of this enzyme bacteria are unable to synthesize the tetrahydrofolic acid, which is an essential precursor in the de novo synthesis of the intermediate Thymidine monophoshpate (dTMP). The dTMP, in turn, is a precursor of DNA metabolite Thymidine triphosphate. Therefore, trimethoprim interferes with bacterial de novo DNA synthesis (Amyes, 1974). Works cited: Ozen H. 'Glycogen storage diseases: new perspectives'. World J Gastroenterol. 2007 May 14;13(18):2541-53. R. Bowen. 'The Endocrine Pancreas: Introduction and Index, Physiologic Effects of Insulin'. http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/pancreas/index.html, 2007 David J. Kelly and Nicky J. Hughes. "Chapter 12-The Citric Acid Cycle and Fatty Acid Biosynthesis". Helicobacter Pylori physiology and genetics. Ed: Harry.L.T Mobley, George L.Mendez and Stuart L. Hazell. ASM press, 2001. Pitson SM, Mendz GL, Srinivasan S, Hazell SL. 'The tricarboxylic acid cycle of Helicobacter pylori'. Eur J Biochem. 1999 Feb;260(1):258-67. Hitoshi Tsugawa et al; 'Alpha-ketoglutarate oxidoreductase, an essential salvage enzyme of energy metabolism, in coccoid form of Helicobacter pylori'. Biochemical and Biophysical Research Communications 2008 Nov 7, Vol 376 (1): 46-51. Zhongming Ge. 'Potential of fumarate reductase as a novel therapeutic target in Helicobacter pylori infection' Expert Opinion on Therapeutic Targets. 2002 april, Vol 6(2); 135-146. DOI 10.1517/14728222.6.2.135 Chia-Wei Cheung, Natalie S. Cohen, and Luisa RaijmanS 'Channeling of Urea Cycle Intermediateisn Situ in Permeabilized Hepatocytes' The Journal of Biological Chemistry , 1989 march 5. Vol 264 (7); 4038-4044. Karl S Roth 'N-Acetylglutamate Synthetase Deficiency'. e-medicine, http://emedicine.medscape.com/article/941090-overview. Updated: Aug 4, 2008 Ljubica Caldovic, Giselle Y. Lopez, Nantaporn Haskins, Maria Panglao, Dashuang Shi, Hiroki Morizono and Mendel Tuchman 'Biochemical properties of recombinant human and mouse N-acetylglutamate synthase'. Molecular Genetics and Metabolism, 2006. Vol 87 (3); 226-232. Joseph Dancis Festschrift. 'Maple syrup urine disease: It has come a long way'. Journal of Pediatrics 1998 march, 132:3S Ralph Fingerhut, Eva Simon, Esther M. Maier, Julia B. Hennermann and Udo Wendel 'Maple Syrup Urine Disease: Newborn Screening Fails to Discriminate between Classic and VariantForms'.ClinicalChemistry.2008;54:1739-1741. Puffenberger E.G. 'Genetic heritage of the Old Order Mennonites of southeastern Pennsylvania. American Journal of Medical Genetics Part C: Seminars in Medical Genetics 2003. Volume 121C(1),18-31 Qasim Waseem, H. Bobby Gaspar and Adrian J. Thrasher. 'Biochemical defects in adenosine deaminase(ADA)deficiency'.ExpertReviewsinMolecularMedicine: http://www.expertreviews.org/: 2004, july. Vol. 6 (13); FAIRBANKS L.D, C. L. SHOVLIN, A. D. B. WEBSTER, J. M. B. HUGHES and H. A. SIMMONDS 'Adenosine Deaminase Deficiency with Altered Biochemical Parameters in Two Sisters with Late-onset Immunodeficiency' J. lnher. Metab. Dis. 1994. Vol 17; 135-137 AMYES S. G. B. AND J. T. SMITHTm 'Trimethoprim Action and Its Analogy with Thymine Starvation' ANTIMICROBIAL AGENTS AND CHEMOTHERAPY, Feb. 1974, Vol 5 (2)p. 169-178 Read More