MOLECULAR-BASIS OF ESSENTIAL FRUCTOSURIA - MOLECULAR-CLONING AND MUTATIONAL ANALYSIS OF HUMAN KETOHEXOKINASE (FRUCTOKINASE)

Essential fructosuria is one of the oldest known inborn errors of meta bolism. It is a benign condition which is believed to result from defi ciency of hepatic fructokinase (ketohexokinase, KHK, E.C.2.7.1.3). Thi s enzyme catalyses the first step of metabolism of dietary fructose, c onversion of fructose to fructose-1-phosphate. Despite the early recog nition of this disorder, the primary structure of human KHK and the mo lecular basis of essential fructosuria have not been previously define d. In this report, the isolation and sequencing of full-length cDNA cl ones encoding human ketohexokinase are described. Alternative mRNA spe cies and alternative KHK isozymes are produced by alternative polyaden ylation and splicing of the KHK gene. The KHK proteins show a high lev el of sequence conservation relative to rat KHK. Direct evidence that mutation of the KHK structural gene is the cause of essential fructosu ria was also obtained. In a well-characterized family, in which three of eight siblings have fructosuria, all affected individuals are compo und heterozygotes for two mutations Gly40Arg and Ala43Thr. Both mutati ons result from G-A transitions, and each alters the same conserved re gion of the KHK protein. Neither mutation was seen in a sample of 52 u nrelated control individuals. An additional conservative amino acid ch ange (Val49lle) was present on the KHK allele bearing Ala43Thr.