Fructokinase from rat liver. II. The role of K+ on the enzyme activity

Expression of rat liver ketohexokinase in yeast results in fructose intolerance

Rat liver ketohexokinase (ATP:D-fructose 1-phosphotransferase; EC 2.7.1.3) was purified to homogeneity and the molecular mass of the protein was found by mass spectrometry to be 32,800 Da. The enzyme was cleaved and the amino acid sequences of seven peptides, comprising 24% of the total sequence, were determined. This sequence information was used to design oligonucleotide primers for a PCR using rat liver single-stranded cDNA as a template. The 224 bp PCR product was used as a probe to screen a rat liver cDNA library. A cDNA sequence of 1342 bp was obtained from three positive clones. This contained the entire coding region for ketohexokinase, and all seven peptides were identified in the predicted amino acid sequence. When ketohexokinase was expressed in Saccharomyces cerevisiae using the yeast expression vector pMA91, the cells became intolerant of the presence of fructose in their growth media. The growth of an exponential-phase culture was completely arrested within 90 min by the addition of fructose to a final concentration as low as 0.1% (w/v). This response is associated with an accumulation of fructose 1-phosphate. The cDNA for ketohexokinase encodes a protein composed of 299 amino acids with a combined molecular mass of 32,728 Da. This is in close agreement with the value for the isolated protein determined by mass spectrometry. The primary structure does not show any significant homology with those of other eukaryotic hexokinases, but it contains a highly conserved region that is present in three prokaryotic phosphotransferases that have furanose substrates.

The purification and properties of human liver ketohexokinase. A role for ketohexokinase and fructose-bisphosphate aldolase in the metabolic production of oxalate from xylitol

Ketohexokinase (EC 2.7.1.3) was purified to homogeneity from human liver, and fructose-bisphosphate aldolase (EC 4.1.2.13) was partially purified from the same source. Ketohexokinase was shown, by column chromatography and polyacrylamide-gel electrophoresis, to be a dimer of Mr 75000. Inhibition studies with p-chloromercuribenzoate and N-ethylmaleimide indicate that ketohexokinase contains thiol groups, which are required for full activity. With D-xylulose as substrate, ketohexokinase and aldolase can catalyse a reaction sequence which forms glycolaldehyde, a known precursor of oxalate. The distribution of both enzymes in human tissues indicates that this reaction sequence occurs mainly in the liver, to a lesser extent in the kidney, and very little in heart, brain and muscle. The kinetic properties of ketohexokinase show that this enzyme can phosphorylate D-xylulose as readily as D-fructose, except that higher concentrations of D-xylulose are required. The kinetic properties of aldolase show that the enzyme has a higher affinity for D-xylulose 1-phosphate than for D-fructose 1-phosphate. These findings support a role for ketohexokinase and aldolase in the formation of glycolaldehyde. The effect of various metabolites on the activity of the two enzymes was tested to determine the conditions that favour the formation of glycolaldehyde from xylitol. The results indicate that few of these metabolites affect the activity of ketohexokinase, but that aldolase can be inhibited by several phosphorylated compounds. This work suggests that, although the formation of oxalate from xylitol is normally a minor pathway, under certain conditions of increased xylitol metabolism oxalate production can become significant and may result in oxalosis.

The mechanism of guanosine triphosphate depletion in the liver after a fructose load. The role of fructokinase

A Sephadex G-25 filtrate of a 100 000g supernatant of rat liver homogenate was shown to be able to phosphorylate fructose, with GTP as the phosphate donor. Attempts to separate ATP- and GTP-dependent fructokinase activities failed, indicating that there is a single enzyme able to use both nucleotides. With a partially purified enzyme, Km values for fructose of 0.83 and 0.56 mM were found with ATP and GTP as substrates respectively. Km values of 1.53 and 1.43 mM were found for GTP and ATP respectively. Both ADP and GDP inhibited the GTP- and ATP-dependent fructokinase activity. We conclude that the depletion of hepatic GTP caused by intravenous administration of fructose to mice and rats can be explained simply by the utilization of the nucleotide by fructokinase.

Evidence that the severity of depletion of inorganic phosphate determines the severity of the disturbance of adenine nucleotide metabolism in the liver and renal cortex of the fructose-loaded rat

To test the hypothesis that in both the liver and renal cortex of the fructose-loaded rat, severity of depletion of inorganic phosphate (P(i)), and not the magnitude of accumulation of fructose-1-phosphate (F-1-P), determines the severity of the dose-dependent reduction of ATP, we intraperitoneally injected fed rats with fructose, 20 and 40 mumol/g, alone, and at the higher load, in combination with (a) sodium phosphate, 20 mumol/g, administered shortly beforehand or subsequently or, (b) adenosine, 2 mumol/g, administered beforehand. The following observations were made: (a) With fructose loading alone, at the higher load, both P(i) and total adenine nucleotides (TAN) were reduced by one third in the renal cortex and (as previously observed) by two thirds in the liver; and at either load, the reduction of ATP (and TAN) and the accumulation of F-1-P were less severe in the renal cortex than in the liver. (b) Prior phosphate loading largely prevented the reductions of ATP and TAN in the renal cortex and significantly attenuated them in the liver, yet doubled the renal cortical accumulation of F-1-P. (c) Adenosine loading substantially attenuated the reductions of ATP, TAN, and P(i) only in the renal cortex. (d) ATP varied directly with P(i) (P < 0.001, r = 0.98) in the domain of control and reduced values of P(i) taken from both liver and renal cortex. (e) As judged from tissue and plasma concentrations of fructose and glucose, and tissue concentrations of fructose-6-phosphate and glucose-6-phosphate, the rate at which the renal cortex and liver converted fructose to glucose was much lower at the higher fructose load. (f) Prior phosphate loading prevented this decrease in rate in the renal cortex and attenuated it in the liver; adenosine loading attenuated it only in the renal cortex. We conclude that in both the renal cortex of the fructose-loaded rat: (a) Depletion of P(i) is critical to the causation of the reductions in both ATP and TAN and, at the higher fructose load, of a decrease in the rate at which ATP is regenerated. (b) The severity of depletion of P(i) determines the severity of these disturbances. (c) By differentially mitigating the depletion of P(i), prior phosphate loading largely prevents these disturbances in the renal cortex, and attenuates them in the liver; and adenosine loading attenuates them only in the renal cortex. The findings provide some basis for the observation that in patients with hereditary fructose intolerance experimentally exposed to fructose, prior loading with sodium phosphate substantially attenuates the renal but not hepatic dysfunction.