This study assessed the changes in digoxin and some other metabolites of the isoprenoid pathway in metabolic syndrome X presenting with multiple lacunar state. The isoprenoid pathway and digoxin status was also studied for comparison in individuals of differing hemispheric dominance to find out the role of cerebral dominance in the genesis of syndrome X. There was an increase in plasma HMG CoA reductase activity with a consequent increase in serum digoxin, which caused a reduction in RBC membrane Na^sup +^-K^sup +^ ATPase activity. There was an increase in serum tryptophan and its metabolites and a decrease in tyrosine and its metabolites. Serum magnesium was decreased with consequent alteration in the metabolism of glycosaminoglycans and glycolipids. Increase in dolichol, another product of the isoprenoid pathway, resulted in alteration in glycoprotein metabolism. Changes in the composition of membrane glycosaminoglycans, glycoproteins and cholesterol : phospholipid ratio were also observed in this disorder leading to decreased lysosomal stability. Decrease in ubiquinone, another isoprenoid metabolite, resulted in alteration in the free radical generation. Membrane Na^sup +^-K^sup +^ ATPase inhibition due to digoxin, altered membrane structure, increased tryptophan catabolites and decreased tyrosine catabolites can lead to increased intracellular calcium and reduced intracellular magnesium which can account for the symptoms of syndrome X. The biochemical patterns including hyperdigoxinemia observed in syndrome X correlated with those obtained in right hemispheric chemical dominance. Right hemispheric chemical dominance is a predisposing factor for syndrome X with multiple lacunar state. [Neurol Res 2003; 25: 739-744]
Keywords: Digoxin; dolichol; ubiquinone; hemispheric dominance; syndrome X
The components of syndrome X include noninsulin-dependent diabetes mellitus, hyperinsulinism, insulin resistance, central obesity, dyslipidaemia marked by hypertriglyceridemia and low HDL levels, accelerated atherosclerosis leading to coronary artery disease and stroke, hypertension and a positive family history. The isoprenoid pathway produces four crucial metabolites important in cellular function - digoxin, an endogenous inhibitor of membrane Na^sup +^-K^sup +^ ATPase produced by hypothalamus, ubiquinone, a component of the mitochondrial electron transport chain, dolichol, important in N-glycosylation of proteins and cholesterol, a component of cellular membrane. Elevated levels of digoxin and the related increased Na^sup +^-Ca^sup ++^ exchange in the vascular smooth muscle cell has been reported1 to cause the hypertension associated with syndrome X. The inhibition of membrane Na^sup -^-K^sup +^ ATPase by digoxin has been reported2 to cause hypomagnesemia, a risk factor in syndrome X. Magnesium deficiency has been associated with insulin resistance3. Hypomagnesemia can also affect the metabolism of glycosaminoglycans and glycolipids and changes in the dolichol levels can alter N-glycosylation of protein. Changes in basement membrane heparan sulphate has been implicated in the microangiopathy of syndrome X and elevated levels of sialic acid, an acute phase response marker has also been documented4 in syndrome X. The acute phase response plays a role in the genesis of the vascular disease in syndrome X. Digoxin induced altered calcium/magnesium ratios and changes in ubiquinone can affect mitochondrial function and lead to free radical generation. Free radicals can contribute to oxidised LDL important in the pathogenesis of atherosclerosis in syndrome X5. Alteration in baroreceptor sensitivity and sympatho-vagal balance has been reported6 to lead to vasospasm in syndrome X. Digoxin can alter amino acid and neurotransmitter transport7. The products of the isoprenoid pathway - cholesterol, ubiquinone, dolichol and digoxin - can affect membrane structure and function, with consequent endothelial dysfunction important in syndrome X8. The study was undertaken to assess 1. the isoprenoid pathway, 2. the tryptophan/tyrosine catabolic patterns, 3. glycoconjugate metabolism, 4. RBC membrane changes as a reflection of cell membrane change, 5. free radical metabolism. A hypothesis implicating membrane Na^sup +^-K^sup +^ ATPase inhibition consequent to increased digoxin secretion as pivotal to all these changes occurring in syndrome X is also presented. Since digoxin can regulate multiple neurotransmitter systems it could possibly play a role in the genesis of cerebral dominance. The isoprenoid pathway and digoxin status was studied in individuals of differing hemispheric dominance in order to elucidate the role of cerebral dominance in the pathogenesis of syndrome X. The hypothesis tested was - hemispheric chemical dominance mediated by hypothalamic digoxin is crucial to the pathogenesis of syndrome X.
MATERIALS AND METHODS
Each patient/normal individual was informed about the aim of the study and their consent obtained. Necessary ethical clearance was obtained for the study from the ethical committee of Medical College, Trivandrum. Fifteen cases of syndrome X with multiple lacunar state attending the Metabolic and Genetics Department of Medical College Hospital, Trivandrum were chosen at random for the study. The clinical characteristics of the patient population were identical and are as follows:
1. All of them were males between the ages of 45 and 55 years.
2. Had trunkal obesity and type 2 noninsulin-dependent diabetes mellitus with age of onset above 40 years.
4. Hyperinsulinemia and dyslipidemia with low HDL cholesterol/elevated serum triglycerides.
5. History of recurrent strokes.
6. The neurological deficits include dementia of the frontotemporal type/pseudobulbar palsy with dysphagia, dysarthria and dysphonia and bilateral pyramidal dysfunction/extrapyramidal syndrome with cogwheel rigidity and tremor.
7. CT scan evidence of periventricular white matter infarcts suggestive of perforating artery disease.
Each patient had an age and sex matched healthy control selected at random from the general population. All patients and control subjects were nonsmokers (passive or active). The patients chosen for the study were not on digoxin or any similar drug.
Fifteen normal healthy male individuals (45-55 years of age) each of left handed/right hemispheric dominant, right handed/left hemispheric dominant and ambidextrous/bihemispheric dominant individuals diagnosed by the dichotic listening test were chosen for the study. This group was chosen at random from the general population of Trivandrum city. These individuals were not on any drugs like digoxin and were free from any systemic disease. All individuals in this group also were nonsmokers (passive or active).
Analytical procedures used were as follows. All biochemicals used in this study were obtained from Sigma Chemicals, St. Louis, MO, USA. The methodology used for the following parameters: 1. RBC Na^sup +^-K^sup +^ ATPase activity and serum magnesium, 2. HMG CoA reductase, digoxin, ubiquinone, cholesterol and dolichol, 3. tryptophan, tyrosine, serotonin, catecholamines, quinolinic acid, morphine, strychnine and nicotine, 4. total and individual GAG, carbohydrate components of glycoproteins, activity of GAG degrading enzymes and glycohydrolases, 5. free radical scavenging enzymes - superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase9-12.
Statistical analysis was done by ANOVA.
The activity of HMG CoA reductase and the concentration of digoxin and dolichol were increased in syndrome X with multiple lacunar state. The concentration of serum ubiquinone, the activity of erythrocyte membrane Na^sup +^-K^sup +^ ATPase and serum magnesium were decreased. The concentration of serum tryptophan, quinoline acid and serotonin was increased in the plasma of these patients while that of tyrosine, dopamine and noradrenaline was decreased. Nicotine and strychnine were detected in the plasma of syndrome X with multiple lacunar state patients but were not detectable in control serum. Morphine was not detected in the plasma of these patients (Table 1).
The concentration of total glycosaminoglycans (GAG) and different GAG fractions, total hexose, fucose and sialic acid content of serum glycoproteins and the concentration of gangliosides, glycosyl-diglycerides and sulphatides showed significant increase in the serum of syndrome X with multiple lacunar state patients. The activity of glycosaminoglycan (GAG) degrading enzymes and glycohydrolases increased in the serum of syndrome X with multiple lacunar state. The concentration of total GAG and bexose and fructose residues of glycoproteins in the RBC membrane decreased significantly in syndrome X with multiple lacunar state. The concentration of RBC membrane cholesterol increased while that of phospholipids decreased resulting in an increased cholesterol: phospholipid ratio (Table 2).
The activity of superoxide dismutase (SOD), catalase, glutathione reductase and glutathione peroxidase in the erythrocytes decreased significantly in syndrome X with multiple lacunar state. The concentration of nitric oxide (NO) increased significantly while the concentration of reduced glutathione decreased (Table 2).
The results showed that HMG CoA reductase activity, serum digoxin and dolichol were increased and ubiquinone reduced in left handed/right hemispheric dominant individuals. The results also showed that HMG CoA reductase activity, serum digoxin and dolichol were decreased and ubiquinone increased in right handed/left hemispheric dominant individuals. The results showed that the concentration of tryptophan, quinolinic acid serotonin, strychnine and nicotine was found to be higher in the plasma of left handed/right hemispheric dominant individuals while that of tyrosine, dopamine, morphine and norepinephrine was lower. The results also showed that the concentration of tryptophan, quinolinic acid serotonin, strychnine and nicotine was found to be lower in the plasma of right handed/left hemispheric dominant individuals while that of tyrosine, dopamine, morphine and norepinephrine was higher (Table 3).
The increase in plasma digoxin and dolichol in syndrome X is a consequence of increased operation of the isoprenoid pathway as is evidenced from the increase in the activity of HMG CoA reductase. Incorporation of ^sup 14^C-acetate into digoxin in rat brain has been previously shown by us indicating its synthesis in mammals from acetyl CoA and by the isoprenoid pathway13. The observed inhibition of RBC membrane Na^sup +^-K^sup +^ ATPase is a consequence of increased digoxin. The inhibition of Na^sup +^-K^sup +^ ATPase by digoxin is known to cause increase in intracellular Ca^sup ++^ and a decrease in intracellular Mg^sup ++2^. Low intracellular Mg^sup ++^ and high intracellular Ca^sup ++^ consequent of Na^sup +^-K^sup +^ ATPase inhibition appear to be crucial to the pathophysiology of syndrome X with multiple lacunar state.
Digoxin is reported to increase tryptophan transport while decreasing tyrosine transport7. The increase in tryptophan and its catabolites - quinolinic acid and serotonin and the decrease in tyrosine and its catabolites - dopamine and norepinephrine now observed in patients with syndrome X may be a reflection of this effect of digoxin on the transport of these amino acids7. Serum of syndrome X with multiple lacunar state showed the presence of strychnine and nicotine, synthesized from tryptophan which is elevated in syndrome X11. Endogenous morphine, synthesized from tyrosine could not be detected in the serum of these patients as their tyrosine levels are low. The decrease in membrane Na^sup +^-K^sup +^ ATPase activity in syndrome X with multiple lacunar state could also be due to the fact that the hyperpolarising neurotransmitters (dopamine, morphine and noradrenaline derived from tyrosine) are reduced and the depolarising neuroactive compounds (serotonin, strychnine, nicotine and quinolinic acid derived from trytophan) are increased. Thus the schizoid neurotransmitter pattern of reduced dopamine, noradrenaline and morphine and increased serotonin, strychnine and nicotine is also observed by us in syndrome X with multiple lacunar state and could predispose to it9. The increase now observed in quinolinic acid, an NMDA agonist can contribute to glutamate excitotoxicity, as in the schizoid state. The elevated levels of serotonin and strychnine is also known to cause an upregulated excitatory NMDA transmission. Increased glutamatergic transmission can potentiate atherogenesis.
Magnesium is required as a co-factor for cell membrane glucose transport3. Hypomagnesemia, consequent to membrane Na^sup +^-K^sup +^ ATPase inhibition can lead to defective cell membrane transport of glucose. Increased intracellular calcium can also lead to the activation of calcineurin signal transduction pathway resulting in T-cell activation and increased levels of TNF alpha, contributing to insulin resistance4. Increased intracellular calcium can activate the G-protein coupled signal transduction of the contra-insulin hormones (growth hormone and glucagon) leading to hyperglycemia. Decrease in intracellular magnesium can block the phosphorylation reactions involved in protein tyrosine kinase receptor activity leading to insulin resistance14. Decrease in intracellular magnesium can lead to inhibition of glycolysis causing defective glucose utilization and hyperglycemia. Increase in intracellular calcium can open the mitochondrial PT pore, disrupt the hydrogen gradient across the inner membrane and block mitochondrial oxidative phosphorylation15. Intracellular magnesium deficiency can also lead to an ATP synthase defect. This leads to defective glucose utilisation. Increase in beta cell calcium can contribute to increased insulin release from beta cells and hyperinsulinemia. Hypomagnesemia has been reported to markedly increase glucose stimulated insulin secretion by the perfused pancreas. Increase in intracellular calcium can activate the G-protein coupled angiotensin receptor producing hypertension and G-protein coupled thrombin receptor and platelet activating factor receptor producing the thrombosis observed in syndrome X with multiple lacunar state16. Decreased intracellular magnesium can lead to increased thrombin and ADP/collagen induced platelet aggregation. Na^sup +^-K^sup +^ ATPase inhibition related increased smooth muscle calcium and decreased magnesium can contribute to vasospasm and ischemia observed in stroke and CAD. Decreased intracellular magnesium can produce an endothelial mitochondrial dysfunction, increased membrane fluidity of endothelium and increased permeability of endothelial cells to lipoproteins8. Decreased intracellular magnesium can produce dysfunction of lipoprotein lipase producing defective catabolism of triglyceride rich lipoproteins and hypertriglyceridemia. In hypomagnesemia, lecithin cholesterol acyl transferase (LCAT) is defective and there is reduced formation of cholesterol esters in HDL. This results in reduced HDL cholesterol described in syndrome X with multiple lacunar state. Magnesium deficiency has been reported to increase LDL cholesterol levels also.
The magnesium depletion can upregulate the synthesis of glycosaminoglycans and glycoproteins. The elevation in the level of dolichol may suggest its increased availability for N-glycosylation of proteins. The increase in the carbohydrate components total hexose, fucose and sialic acid in syndrome X with multiple lacunar state was not to the same extent, suggesting a qualitative change in glycoprotein structure. The increase in the activity of glycohydrolases and GAG degrading enzymes could be due to reduced lysosomal stability and consequent leakage of lysosomal enzymes into the serum. The increase in the concentration of carbohydrate components of glycoproteins and GAG in spite of increased activity of many glycohydrolases, may be due to their possible resistance to cleavage by glycohydrolases consequent to qualitative change in their structure. Increased levels of sialic acid and ICAM-1, as well as basement membrane heparan sulphate has been reported in syndrome X with multiple lacunar state, suggesting changes in glycoconjugate metabolism related to micro/macroangiopathy of syndrome X17. Increased glycosaminoglycans in the arterial wall can lead on to increased interaction between GAG and lipoproteins contributing to atherogenesis. A number of fucose and sialic acid-containing natural ligands are involved in adhesion of the lymphocyte, producing leukocyte trafficking and extravasation in to the perivascular space contributing to the acute phase response of syndrome X18. The alteration in the isoprenoid pathway can affect cellular membranes. The upregulation of the isoprenoid pathway can lead to increased cholesterol synthesis and magnesium deficiency can inhibit phospholipid synthesis leading to increased membrane cholesterol : phospholipid ratio. The membrane trafficking depends upon GTPases and lipid kinases which are crucially dependent on magnesium and are defective in magnesium deficiency19. The glycoconjugate is therefore defectively incorporated into the cell membrane in syndrome X. The change in membrane structure can produce 1. defective transport of glucose across cell membranes due to alteration in the configuration of the glucose transporter, 2. defective lysosomal stability and leakage of glycohydrolases and GAG degrading enzymes into the serum, 3. defective peroxisomal membranes leading to catalase dysfunction, 4. alteration in endothelial cell membrane contributing to the endothelial dysfunction described in the vascular disease in syndrome X8.
The concentration of ubiquinone decreased significantly in syndrome X with multiple lacunar state which may be the result of low tyrosine levels, consequent to digoxin's effect in preferentially promoting tryptophan transport over tyrosine7. The aromatic ring portion of ubiquinone is derived from tyrosine. Ubiquinone is an important component of the mitochondrial electron transport chain and membrane antioxidant. Ubiquinone deficiency leads to increased generation of free radicals. The increase in intracellular calcium can open up the mitochondrial PT pore causing a collapse of the hydrogen gradient across the inner membrane and uncoupling of the respiratory chain16. Intracellular magnesium deficiency can lead to a defect in the function of ATP synthase. All this leads to defects in mitochondrial oxidative phosphorylation, incomplete reduction of oxygen and generation of superoxide ion which produces lipid peroxidation. The increase in intracellular calcium may lead to increased generation of NO by inducing the enzyme nitric oxide synthase which combines with superoxide radical to form peroxynitrite. Increased calcium also can activate phospholipase A^sub 2^ resulting in increased generation of arachidonic acid which can undergo increased lipid peroxidation. Catalase dysfunction due to defective peroxisomal membranes, superoxide dismutase deficiency consequent to leakage from the intracellular calcium mediated open mitochondrial PT pore and magnesium deficiency related dysfunction of the glutathione system of free radical scavenging can contribute to increased level of free radicals in syndrome X. Increased free radical generation can result in the formation of oxidised LDL which is atherogenic. Oxidised LDL is toxic to vascular endothelium and the macrophage20.
Syndrome X with multiple lacunar state can thus be considered as due to hypothalamic digoxin hypersecretion consequent to a upregulated isoprenoid pathway. The biochemical patterns obtained in syndrome X correlated with those obtained in right hemispheric chemical dominance. In left handed/right hemispheric chemically dominant individuals there was a derangement of the isoprenoid pathway. They had an upregulated HMG CoA reductase activity with increased digoxin and dolichol levels and reduced ubiquinone levels. The RBC membrane Na^sup +^-K^sup +^ ATPase activity was reduced and serum magnesium depleted. The left handed/right hemispheric chemically dominant individuals had increased levels of tryptophan, serotonin, quinolinic acid, strychnine and nicotine while the levels of tyrosine, dopamine, noradrenaline and morphine were lower. Thus an upregulated isoprenoid pathway, increased level of tryptophan and its catabolites, decreased levels of tyrosine and its catabolites and hyperdigoxinemia is suggestive of right hemispheric chemical dominance. In right handed/left hemispheric chemically dominant individuals the biochemical patterns were reversed. Syndrome X occurs in right hemispheric chemically dominant individuals and could be a reflection of altered brain function. Right hemispheric chemical dominance can contribute to a dysregulated isoprenoid pathway and the vascular disease and insulin resistance of syndrome X.
1 Kramer HJ, Meyer Lehnert H, Michel H, Predel HG. Endogenous natriuretics and ouabain like factors and their roles. Am J Hypertens 1991; 1: 81-89
2 Haga H. Effects of dietary magnesium supplementation on diurnal variation of BP and plasma Na^sup +^-K^sup ^+ ATPase activity in essential hypertension, Jpn Heart J 1992; 33: 785-798
3 Paolisso G, Babagallo M. Hypertension, diabetes mellitus, and insulin resistance: The role of intracellular magnesium. Am J Hypertens 1997; 10: 346-355
4 Pickup JC, Mattock MB, Chusney GD, Burt S. NIDDM as a disease of the innate immune system: Association of acute-phase reactants and interleukin-6 with metabolic syndrome X. Diabetologia 1997; 40: 1286-1292
5 Fuster V, Badimon L, Badimon JJ, Chesebro JH. The pathogenesis of coronary artery disease and the acute coronary syndrome. N Engl J Med 1992; 326: 310-318
6 Adamopoulos S, Rosano GM, Ponikowski P, Cerquetani E, Piepoli M, Panagiota F, Collins P, Poole-Wilson P, Kremastinos D, Coats AG. Impaired baroreflex sensitivity and sympathovagal balance in syndrome X. Am J Cardiol 1998; 82: 862-868
7 Hisaka A, Kasamatu S, Takenaga N. Absorption of a novel prodrug DOPA. Drug-Metab Disp 1990; 18: 621-625
8 Bellamy MF, Goodfellow J, Tweddel AC, Dunstan FD, Lewis MJ, Henderson AH, Syndrome X and endothelial dysfunction. Cardiovasc Res 1998; 40: 410-417
9 Ravi Kumar A, Deepa Devi KV, Arun P, Manojkumar V, Kurup PA. Tryptophan and tyrosine catabolic pattern in neuropsychiatric disorders. Neuro Ind 2001; 48: 231-238
10 Ravi Kumar A, Arun P, Deepa Devi KV, Augustine J, Kurup PA. Isoprenoid pathway and free radical generation and damage in neuropsychiatric disorders. Ind J Exp Biol 2000; 38: 438-446
11 Arun P, Ravi Kumar A, Leelamma S, Kurup PA. Endogenous alkaloids in the brain of rats loaded with tyrosine/tryptophan and in the serum of patients of neurodegenerative and psychiatric disorders. Ind J Med Res 1998; 107: 231-238
12 Manoj AJ, Kurup PA. Changes in the glycosaminoglycans and glycoproteins in the rat brain during protein calorie malnutrition. Clin Biochem Nutr 1998; 25: 149-157
13 Ravi Kumar A, Jyothi A, Kurup PA. ^sup 14^C-acetate incorporation into digoxin in rat brain and effect of digoxin administration. Ind J Exp Biol 2001; 3: 420-426
14 Stefan C, Wera S, Stalmans W, Bollen M. The inhibition of insulin receptor by the receptor protein PC is not specific and result from hydrolysis of ATP. Diabetes 1996; 45: 980-986
15 Green DR, Reed JC. Mitochondria and apoptosis. Science 1998; 281: 1309-1316
16 Lansberg L, Young JB. Physiology and pharmacology of ANS. In: Fauci AS, ed. Harrison's Principles of Internal Medicine, New York: McGraw-Hill, 1998: pp. 433-434
17 Ceiello A, Falleti E, Motz E, Taboga C, Tonutti B, Ezsol Z, Gonano F, Bartoli E. Hyperglycemia induced circulating ICAM-1 increase in diabetes mellitus. The possible role of oxidative stress. Horm Metab Res 1998; 30: 146-149
18 Linstinsky JL, Siegal GP, Linstinsky MC. Alpha-L-Fucose a potentially critical molecule in pathologic processes including neoplasia. Am J Clin Pathol 1998; 110: 425
19 Wiedemann C, Cockcroft S. Vesicular transport. Nature 1998; 394: 426
20 Falk E. Why do plaques rupture? Circulation 1992; 86 (Suppl. Ill): 30-42
Ravi Kumar Kurup* and Parameswara Achutha Kurup[dagger]
* Department of Neurology, Medical College, Trivandrum, Kerala
[dagger] Metabolic Disorders Research Center, Trivandrum, Kerala, India
Correspondence and reprint requests to: Dr P.A. Kurup, Gouri Sadan, T.C.4/1525, North of Cliff House, Kattu Road, Kowdiar P.O., Trivandrum, Kerala, India, [email@example.com] Accepted for publication April 2003.
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