The isolation of L-arabitol from pentosuric urine.

The nature of the metabolic defect in congenital pentosuria has not been determined, although considerable evidence is accumulating in regard to the metabolism of L-xylulose. This pentose, long known to be excreted in gm. quantities by pentosuric individuals, has been found in small amounts in the urine of “non-pentosuric” humans (2-4), guinea pigs (3), and rats (4). Touster, Mayberry, and McCormick (5) have demonstrated the conversion of n-glucuronolactone-l-Cl3 to L-xylulose-5-W in a pentosuric human, confirming earlier suggestions in regard to the biosynthesis of the pentose (3). Moreover, three groups of investigators (6-8) have reported the enzymatic transformation of n-glucuronic acid, or of its metabolic product L-gulonic acid, to L-xylulose. Evidence regarding the metabolic disposition of L-xylulose was obtained by Touster et al. (g-11), who found that guinea pig liver contains enzymes which can interconvert n-xylulose and xylitol and also xylitol and Dxylulose. The report of Hickman and Ashwell (12) on the occurrence of a n-xylulose kinase in calf liver which catalyzes the formation of n-xylulose 5-phosphate completed a connection between L-xylulose and an intermediate in the 6-phosphogluconate oxidation pathway (pentose phosphate pathway). Accordingly, the following sequence of reactions appears to occur in mammals: n-glucose -+ n-glucuronate ---, L-gulonate -+ L-xylulose + xylitol --+ n-xylulose + n-xylulose 5-phosphate + hexose phosphate, etc. Evidence obtained in vivo for the position of xylitol in the sequence has been provided by the demonstration that xylitol-l-Cl4 yields a labeling pattern in liver glycogen which is in accord with the metabolism of the pentitol via the pentose phosphate pathway (13). It would be reasonable to assume that the defect in pentosuria is due to a deficiency of the L-xylulose-xylitol enzyme. However, if the metabolic block were between xylitol and n-xylulose, both xylitol and L-xylulose might accumulate and be excreted. Previous analytical studies on pento-