THE COENZYME FUNCTION OF THIAMINE PYROPHOSPHATE IN PENTOSE PHOSPHATE METABOLISM

Thiamine is a water-soluble vitamin acting in the mitochondria as a cofactor for energy metabolism and, in the cytoplasm, in the pentose phosphate biosynthetic pathway. Its transport through the plasma membrane requires two transporters with overlapping functions: THTR1 encoded by SLC19A2, and THTR2 encoded by SLC19A3. Thiamine is transformed into its active form, thiamine pyrophosphate (TPP) by a kinase encoded by the TPK1 gene. Then it may enter the mitochondria through a TPP transporter encoded by SLC25A19. Mutations in SLC19A2 cause thiamine-responsive megaloblastic anemia (TRMA). Mutations in SLC19A3 cause biotin/thiamine–responsive basal ganglia disease. Mutations in SLC25A19 may cause early microcephaly with death in infancy (also called Amish microcephaly) or a later-onset bilateral striatal necrosis with progressive peripheral neuropathy. Recently, mutations in the TPK1 gene have been associated with recurrent encephalopathy with mild lactic acidosis.

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Pentose Phosphate Metabolism of Potato Tuber Discs as Influenced by Prior Storage Temperature

PLANT PHYSIOLOGY ◽

10.1104/pp.61.2.252 ◽

1978 ◽

Vol 61 (2) ◽

pp. 252-253 ◽

Cited By ~ 12

Author(s):

Robert B. Dwelle ◽

Gilbert F. Stallknecht

Keyword(s):

Potato Tuber ◽

Storage Temperature ◽

Pentose Phosphate ◽

Phosphate Metabolism

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THE PENTOSE PHOSPHATE METABOLIC PATHWAY IN THE HUMAN ERYTHROCYTE: II. THE TRANSKETOLASE AND TRANSALDOLASE ACTIVITY OF THE HUMAN ERYTHROCYTE

Canadian Journal of Biochemistry and Physiology ◽

10.1139/o61-053 ◽

1961 ◽

Vol 39 (3) ◽

pp. 533-543 ◽

Cited By ~ 9

Author(s):

Y. S. Brownstone ◽

O. F. Denstedt

Keyword(s):

Human Erythrocyte ◽

Soluble Fraction ◽

Pentose Phosphate ◽

Ion Concentration ◽

Thiamine Pyrophosphate ◽

Magnesium Ions ◽

Michaelis Constant ◽

Hydrogen Ion Concentration ◽

Wide Range ◽

Human Red Cells

Transketolase, in the human erythrocyte, is confined to the soluble fraction of the cell. The activation energy for the formation of sedoheptulose-7-phosphate (S-7-P) from pentose phosphate was found to be 11,500 calories and the rate of formation of S-7-P to be directly proportional to the concentration of the enzyme. The Michaelis constant, with ribose-5-phosphate (R-5-P) as the added substrate, was found to be 6 × 10−3 M. The activity of the enzyme is close to the maximum over a wide range of hydrogen ion concentration (pH 7.1 to 8.3) with only a gradual decrease beyond these limits. The transketolase, in the dialyzed stroma-free hemolyzate, is active without the addition of magnesium ions or thiamine pyrophosphate. It is unaffected by sulphydryl-binding inhibitors and by EDTA and oxythiamine pyrophosphate.Transaldolase activity also has been demonstrated in the hemolyzate of human red cells. The rate of the production of hexose phosphate from sedoheptulose-7-phosphate was found to be of the order of 40 μmoles/g Hb/hour. The activity of the enzyme is close to the maximum between pH 7.18 and 7.75.

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New reaction sequences for the non-oxidative pentose phosphate pathway

Biochemical Journal ◽

10.1042/bj1760257 ◽

1978 ◽

Vol 176 (1) ◽

pp. 257-282 ◽

Cited By ~ 53

Author(s):

J F Williams ◽

P F Blackmore ◽

M G Clark

Keyword(s):

Volatile Compounds ◽

Rat Liver ◽

Pentose Phosphate Pathway ◽

Carbon Balance ◽

Pentose Phosphate ◽

Oxidative Pentose Phosphate Pathway ◽

Rat Tissues ◽

Phosphate Metabolism ◽

Reaction Sequence ◽

Triose Phosphate

1. Reactions leading to the formation of 14C-labelled volatile compounds and compounds volatile under acid conditions were investigated in a system actively synthesizing hexose 6-phosphates from [U-14C]ribose 5-phosphate by reactions catalysed by enzymes prepared from acetone-dried powder of rat liver; no reactions involving 14C-labelled volatile compounds were detected. Similarly the fixation of 14C-labelled volatile compounds into hexose 6-phosphate could not be detected. 2. A complete carbon balance was made for the reactants, intermediates and products of the reactions involved in the conversion of ribose 5-phosphate into hexose 6-phosphate by enzymes of rat liver. Five additional intermediates of pentose 5-phosphate metabolism in liver were detected, namely D-manno-heptulose 7-phosphate, D-altro-heptulose 1,7-bisphosphate, D-glycero-D-ido-octulose 1,8-bisphosphate, D-glycero-D-altro-octulose 1,8-bisphosphate and D-arabinose 5-phosphate. 3. D-Arabinose 5-phosphate was found to be utilized by a rat liver enzyme preparation to produce both hexose 6-phosphate and triose phosphate. 4. D-Arabinose 5-phosphate was reversibly converted into other pentose 5-phosphates. Paper chromatographic and enzymic evidence indicated that the conversion involved an enzyme tentatively named arabinose phosphate 2-epimerase, which catalyses the following reaction: D-arabinose 5-P in equilibrium D-ribose-5-P. 5. A variety of rat tissues also utilized D-arabinose 5-phosphate to produce both hexose 6-phosphate and triose phosphate and at a rate comparable with that obtained with D-ribose 5-phosphate. 6. A new reaction sequence for the non-oxidative pentose phosphate pathway in liver is proposed.

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