Thiamine (vitamin B1) is the water-soluble vitamin that is the essential cofactor for the pyruvate dehydrogenase (PDH) complex and for the alpha-ketoglutarate dehydrogenase complex — the two gateway enzymes of the carbohydrate metabolism that connect glycolysis to the TCA cycle and that are therefore essential for the generation of the ATP from the glucose. The PDH complex catalyses the oxidative decarboxylation of the pyruvate to the acetyl-CoA, which is the entry point of the glucose-derived carbon into the TCA cycle, and the alpha-ketoglutarate dehydrogenase complex catalyses the oxidative decarboxylation of the alpha-ketoglutarate to the succinyl-CoA in the TCA cycle. Both of these enzyme complexes require the thiamine pyrophosphate (TPP) as a cofactor — TPP is the coenzyme form of thiamine that is synthesised by the TPP-dependent enzyme thiamine pyrophosphokinase (which uses ATP and thiamine as substrates) and that contains the thiazole ring that is the unique structural feature of the thiamine molecule. Without adequate thiamine and TPP, the PDH complex and the alpha-ketoglutarate dehydrogenase complex cannot function, the pyruvate cannot be converted to acetyl-CoA, the TCA cycle cannot proceed, and the cellular energy metabolism collapses — producing the beri-beri (the cardiovascular and the neurological manifestations of the thiamine deficiency), the Wernicke-Korsakoff syndrome (the neuropsychiatric manifestation of the thiamine deficiency that is most commonly seen in the chronic alcoholics), and the cardiac failure that is the most common cause of death in the severe thiamine deficiency.
The Pyruvate Dehydrogenase Complex and the Carbohydrate Metabolism
The pyruvate dehydrogenase (PDH) complex is a large, multi-enzyme complex that is located in the mitochondrial matrix and that consists of three enzymatic components — the E1 component (pyruvate dehydrogenase, which uses TPP as a cofactor), the E2 component (dihydrolipoamide acetyltransferase, which uses the lipoamide cofactor), and the E3 component (dihydrolipoamide dehydrogenase, which uses FAD and NAD as cofactors). The PDH complex catalyses the five-step reaction that converts the pyruvate to the acetyl-CoA, and it is the gateway through which the carbon from the glucose enters the TCA cycle. The regulation of the PDH complex is achieved by the磷酸化/dephosphorylation of the E1 component (the PDH kinase phosphorylates and inactivates the PDH, and the PDH phosphatase dephosphorylates and activates the PDH) and by the product inhibition by the acetyl-CoA and by the NADH (which are the end products of the PDH reaction and which signal that the energy status of the cell is high). When thiamine is deficient and TPP is not available, the PDH complex cannot function, the pyruvate accumulates, the acetyl-CoA production falls, and the cellular energy metabolism is severely compromised — this is the primary mechanism of the thiamine deficiency.
The clinical importance of the thiamine for the PDH complex function is underscored by the observation that the thiamine deficiency produces a characteristic elevation of the blood pyruvate levels (because the pyruvate cannot be converted to acetyl-CoA) and a reduced activity of the PDH in the tissues (which is the diagnostic marker of the thiamine deficiency). The elevation of the blood pyruvate is one of the most sensitive indicators of the thiamine deficiency, and it is used as a diagnostic test for the thiamine status — the administration of thiamine reverses the elevation of the blood pyruvate and confirms the diagnosis of the thiamine deficiency.
Thiamine and the Wernicke-Korsakoff Syndrome
The Wernicke-Korsakoff syndrome is the neuropsychiatric manifestation of the thiamine deficiency that is most commonly seen in the chronic alcoholics — it is characterised by the triad of the ophthalmoplegia (the paralysis of the eye muscles), the ataxia (the loss of coordination), and the confusion (the acute encephalopathy), and by the anterograde amnesia and the confabulation that are the hallmark of the Korsakoff syndrome (the chronic, irreversible form of the thiamine deficiency brain damage). The Wernicke encephalopathy is the acute, reversible form of the thiamine deficiency brain damage, and it is a medical emergency that requires the immediate administration of the intravenous thiamine (500mg, three times daily for 3-5 days, followed by 250mg daily for the maintenance) to prevent the progression to the Korsakoff syndrome and to the permanent brain damage. The mechanism of the thiamine deficiency brain damage involves the energy failure (because the PDH and the alpha-KGDH cannot function without TPP), the accumulation of the toxic metabolites (including the pyruvate, the lactate, and the alpha-ketoglutarate), and the excitotoxicity (because the impaired energy metabolism leads to the failure of the ion pumps and to the depolarisation of the neurons). These mechanisms are particularly damaging to the brain regions that have the highest metabolic rate (the mammillary bodies, the medial thalamus, and the periventricular regions) — which are the brain regions that are most affected in the Wernicke-Korsakoff syndrome.
Practical Application
For general thiamine supplementation, the evidence-based approach is to supplement with 10-50mg of thiamine daily (as the thiamine hydrochloride or as the thiamine pyrophosphate form, which is the active coenzyme form). The RDA of thiamine is 1.2mg daily for men and 1.1mg daily for women, and it is easily obtained from a varied diet (pork, whole grains, legumes, nuts, seeds). For the treatment of the thiamine deficiency (including the beri-beri and the Wernicke-Korsakoff syndrome), the high-dose thiamine supplementation is required — 100-200mg of thiamine daily is given for the mild deficiency, and 500mg of thiamine is given intravenously three times daily for the severe deficiency (including the Wernicke encephalopathy). For comprehensive carbohydrate metabolism support, thiamine pairs well with the magnesium (which is required for the function of the thiamine pyrophosphokinase enzyme that synthesises the TPP from thiamine), with the other B-complex vitamins (which are required for the function of the other cofactor-dependent enzymes in the carbohydrate metabolism pathway), and with the alpha-lipoic acid (which has a synergistic effect on the glucose metabolism and which is used in the treatment of the diabetic neuropathy).
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