Presence of fructokinase in pancreatic islets

Malaisse, Willy; Malaisse‐Lagae, Francine; Davies, Dewi R.; Schaftingen, Emile Van

doi:10.1016/0014-5793(89)81085-3

Cited by 29 publications

(19 citation statements)

References 23 publications

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“…KHK is believed to be synthesized largely in liver, renal cortex, and small intestine, but one previous report also demonstrated KHK-like activity in pancreas (6). Our preliminary RT-PCR experiments are not quantitative but tend to confirm these observations at the RNA level.…”

Section: Discussionsupporting

confidence: 50%

“…Tissue distribution of KHK mRNA KHK activity is found principally in liver, kidney, and small intestine (2); pancreatic islet cells also possess a similar activity (6). Because of this tissue specificity, it was initially unclear whedier analysis of KHK RNA in patients with essential fructosuria would be possible.…”

Section: Alternative Splicing and Polyadenylationmentioning

confidence: 99%

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Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)

Bonthron¹,

Brady

et al. 1994

Hum Mol Genet

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Essential fructosuria Is one of the oldest known inborn errors of metabolism. It Is a benign condition which is believed to result from deficiency of hepatic fructokinase (ketohexokinase, KHK, E.C.2.7.1.3). This enzyme catalyses the first step of metabolism of dietary fructose, conversion of fructose to fructose-1-phosphate. Despite the early recognition of this disorder, the primary structure of human KHK and the molecular basis of essential fructosuria have not been previously defined. In this report, the isolation and sequencing of full-length cDNA clones encoding human ketohexokinase are described. Alternative mRNA species and alternative KHK isozymes are produced by alternative polyadenylation and splicing of the KHK gene. The KHK proteins show a high level of sequence conservation relative to rat KHK. Direct evidence that mutation of the KHK structural gene Is the cause of essential fructosuria was also obtained. In a wellcharacterized family, in which three of eight siblings have fructosuria, all affected individuals are compound heterozygotes for two mutations Gly40Arg and Ala43Thr. Both mutations result from G-A transitions, and each alters the same conserved region of the KHK protein. Neither mutation was seen In a sample of 52 unrelated control individuals. An additional conservative amlno acid change (Val49lle) was present on the KHK allele bearing Ala43Thr.

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Section: Discussionsupporting

confidence: 50%

Section: Alternative Splicing and Polyadenylationmentioning

confidence: 99%

Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)

Bonthron¹,

Brady

et al. 1994

Hum Mol Genet

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“…Since β cells (believed to contain fructokinase ; [6]) represent only a small proportion of the total pancreatic cell mass, we used RT-PCR to look for lower levels of KHK transcripts and for expression of the 3a exon. After reverse transcription using a poly(dT) containing primer (RACE2), PCR products containing exons 2Ϫ4 were generated using a fluorescein-tagged downstream primer (KHK31f, exon 4) and an unlabelled upstream primer (KHKR2, exon 2).…”

Section: Resultsmentioning

confidence: 99%

“…One particular point which requires further investigation is the nature of a heat-stable fructokinase activity in pancreatic islets [6]. Islet β-cell glucokinase (hexokinase IV, GCK) is known to be central to the regulation of blood glucose levels.…”

mentioning

confidence: 99%

Structure and alternative splicing of the ketohexokinase gene

Hayward

Bonthron

1998

European Journal of Biochemistry

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Ketohexokinase (fructokinase, KHK) catalyses the phosphorylation of fructose to fructose-1-phosphate. It thereby initiates the intracellular catabolism of a large proportion of dietary carbohydrate. Although found at high level in liver, renal cortex and small intestine, fructokinase activity has also been known for many years to be present at lower levels in most other tissues. We previously found that there appeared to be two isoforms of human KHK, and have now investigated the molecular basis for this in human, rat and mouse. Cloning of the human KHK gene, on chromosome 2p23.2-2p23.3, shows that it has nine exons, spanning 14 kb. An intragenic duplication has resulted in two similar 135-bp exons (designated 3a and 3c), separated by a short intron. Exon 3a and exon 3c are mutually exclusively spliced into KHK mRNA. This exonϪintron structure and the pattern of alternative splicing are conserved in both the rat and mouse, suggesting distinct conserved functions for the two KHK isoforms. The alternative splicing is also tissue specific, since in both rat and human, tissues expressing high levels of KHK (liver, kidney and duodenum) utilise exclusively the 3c exon, while other tissues use only 3a. Furthermore, comparison of human foetal and adult tissues indicates a developmental splicing shift from use of exon 3a to exon 3c.Keywords : fructokinase; ketohexokinase; alternative splicing ; isozyme.Ketohexokinase (fructokinase, KHK) catalyses the phosphorylation of fructose to fructose-1-phosphate (F1P). The enzyme can also phosphorylate a variety of other furanose sugars, and the substrate requirements for hepatic KHK have been defined in detail [1]. Dietary fructose (including the large quantity derived from sucrose hydrolysis) is thereby entered into a specialised catabolic pathway involving aldolase B and triokinase. This pathway metabolises fructose to the glycolytic intermediate glyceraldehyde-3-phosphate, in the process bypassing the important regulatory phosphofructokinase/fructose bisphosphatase step of glycolysis. KHK is most abundant in the liver, where it constitutes between 0.04% and 0.6 % of total protein [1,2]. The enzyme has been purified from human [3], bovine [1] and rat liver [2]. In its active form, it is a dimer [3] and requires K ϩ and ATP for activity. The physiological substrate for hepatic KHK is D-fructose, derived from dietary fructose, sucrose, or sorbitol, but it can also phosphorylate D-xylulose [1,3].Although most abundant in the liver, KHK is also found in kidney, small intestine and pancreas [2] and at much lower levels in heart, brain and muscle [3]. The distribution of highexpressing tissues is generally consistent with a major role in

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“…The biochemically well-characterized KHK-C, usually referred to as fructokinase, is encoded by the mRNA that includes exon 3c. It is abundant in liver, renal cortex, and small intestine and also is present in the pancreatic islet (12,13).…”

mentioning

confidence: 99%

Properties of Normal and Mutant Recombinant Human Ketohexokinases and Implications for the Pathogenesis of Essential Fructosuria

et al. 2003

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Alternative splicing of the ketohexokinase (fructokinase) gene generates a "central" predominantly hepatic isoform (ketohexokinase-C) and a more widely distributed ketohexokinase-A. Only the abundant hepatic isoform is known to possess activity, and no function is defined for the lower levels of ketohexokinase-A in peripheral tissues. Hepatic ketohexokinase deficiency causes the benign disorder essential fructosuria. The molecular basis of this has been defined in one family (compound heterozygosity for mutations Gly40Arg and Ala43Thr). Here we show that both ketohexokinase isoforms are indeed active. Ketohexokinase-A has much poorer substrate affinity than ketohexokinase-C for fructose but is considerably more thermostable. The Gly40Arg mutation seems null, rendering both ketohexokinase-A and ketohexokinase-C inactive and largely insoluble. The Ala43Thr mutant retains activity, but this mutation decreases the thermal stability of both ketohexokinase-A and ketohexokinase-C. At physiologic temperature, this results in significant loss of ketohexokinase-C activity but not of ketohexokinase-A. Affected individuals who carry both mutations therefore probably have a selective deficiency of hepatic ketohexokinase, with peripheral ketohexokinase-A being preserved. These findings raise the possibility that ketohexokinase-A serves an unknown physiologic function that remains intact in essential fructosuria. Further mutation analysis in this rare disorder could illuminate the question of whether ketohexokinase-A activity is, unlike that of ketohexokinase-C, physiologically indispensable.

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Presence of fructokinase in pancreatic islets

Cited by 29 publications

References 23 publications

Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)

Molecular basis of essential fructosuria: molecular cloning and mutational analysis of human ketohexokinase (fructokinase)

Structure and alternative splicing of the ketohexokinase gene

Properties of Normal and Mutant Recombinant Human Ketohexokinases and Implications for the Pathogenesis of Essential Fructosuria

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