Human red cell NADP-dependent xylitol dehydrogenase: kinetic and genetic studies
Lane, Anthony Bruce
A deficiency of the enzyme NADP dependent xylitol dehydrogenase (L-xylulose reductase) has previously been found to be the cause of chronic essential pentosuria. Essential pentosuria is a recessively inherited condition which is marked by the continual excretion of relatively large amounts of the enzymes substrate, L-xylulose. The major objective of the study described was to find a simple method for the identification of individuals who are heterozygous for the "pentosuria" and normal alleles. The pentosuria allele could then be used as a gene marker in linkage studies aimed at mapping the L-xylulose reductase locus. A L-xylulose reductase assay suitable for the identification of carriers of essential pentosuria was developed and tested on members of a South African Lebanese family in which the inheritance of pentosuria had previously been suggested to be dominant. It was found that family members could, on the basis of their L-xylulose reductase activities, be classified as either normal, heterozygous or homozygous for the pentosuria allele. Measurements of serum L-xylulose concentrations revealed that pentosuria is, contrary to the previous report, . recessively inherited in this family. A sample of the local Ashkenazi Jewish population was screened for pentosuria carriers. Six out of the 237 individuals screened were found (on the basis of their L-xylulose reductase activities and from the results of a loading test), to carry the pentosuria allele. The frequency of the pentosuria allele in this population was estimated from the apparent heterozygote frequency to be 0.0127. Linkage analyses were carried out on the families of the identified heterozygotes and on members of the Lebanese family mentioned above. No evidence of tight linkage was found between the pentosuria allele's locus and those coding for various red cell antigens, red cell enzymes and serum proteins. Kinetic, chromatographic and electrophoretic studies revealed that the red cells of normal individuals contain two distinct L-xylulose reductases, a minor and a major isozyme. Pentosurics lack the major isozyme but appear to have approximately normal amounts of the minor isozyme. The minor isozyme is e1ectrophoretica 1 1 y distinct from the major isozyme, has markedly higher Michael is constants for the substrates L-xylulose and xylitol and shows a lower pH optimum when catalysing the oxidation of xylitol. Electrophoresis also revealed that liver tissue contains two L-xylulose reductases which occur in similar proportions to those of red cells but which migrate at slightly different rates.
A Thesis submitted to the Faculty of Medicine, University of the Witwatersrand, Johannesburg, for the Degree of Doctor of Philosophy.