Pentosuria

ABSTRACT

• 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.

• 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.

• 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.

• 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.

• 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.

• 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.)

INTRODUCTION

The first six decades that followed the original description by Salkowski and Jastrowitz in 1892 of an individual with essential pentosuria¹ yielded much information concerning the clinical aspects of this condition but virtually none that cast light on the nature of the biochemical lesion. Clinical and laboratory studies in the ensuing 18 years, however, led to identification of the metabolic defect. As a result, and perhaps even more significantly, important information was uncovered concerning the operation in normal individuals of a previously unknown pathway of carbohydrate metabolism. Such an elucidation of a normal mechanism by studies of an accident of nature led Garrod² in 1928 to stress “the lesions of rare maladies,” and prompted Harvey³ 300 years earlier to note that “nature is nowhere accustomed more openly to display her secret mysteries than in cases where she shows traces of her workings apart from the beaten path.”

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