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  1. Essential pentosuria is an inborn error of metabolism in which 1.0 to 4 g of the pentose L-xylulose is excreted in the urine each day. It is a benign disturbance that occurs principally in Jews and behaves genetically as an autosomal-recessive characteristic.

  2. This disorder bears no relationship to diabetes mellitus and is easily distinguished from several other varieties of pentosuria in which milligram quantities of a number of pentoses other than L-xylulose appear in the urine.

  3. Essential pentosuria is the result of a defect in the glucuronic acid oxidation pathway. In this route of carbohydrate metabolism the carboxyl carbon atom of D-glucuronic acid is removed in a series of reactions, giving rise to the pentose L-xylulose. The latter may then be converted to its stereoisomer, D-xylulose, which, in turn, may be phosphorylated. D-xylulose-5-phosphate may participate in reactions of the pentose phosphate pathway which lead to its conversion to hexose phosphate. (Glucuromic acid→gulonic acid→L-xylulose→xylitol→D-xylulose→ pentose phosphate pathway→hexose phosphate.) The glucuronic acid oxidation pathway serves no essential function in humans.

  4. The block results from reduced activity of the nicotinamide adenine dinucleotide phosphate (NADP)-linked xylitol dehydrogenase, the enzyme that catalyzes the conversion of L-xylulose to xylitol.

  5. The heterozygote can be recognized by demonstrating either an intermediate level of erythrocyte activity of xylitol dehydrogenase or increased urinary or serum L-xylulose, or both, in a glucuronolactone loading test.

  6. Two previously unreported inborn errors of metabolism occur in the reversible part of the pentose phosphate pathway. Deficiency of ribose-5-phosphate isomerase has been described in one patient who suffered from a progressive leukoencephalopathy and developmental delay. Transaldolase deficiency has been diagnosed in two unrelated families in whiupdt the proband presented in the newborn period with liver problems. (See Supplement: Ribose-5-Phosphate Isomerase Deficiency and Transaldolase Deficiency.)

This chapter is reprinted from the fourth edition with an addendum by the editors as there have not been substantial developments in the area.

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