Fructose andd‐Glyceraldehyde Metabolism in the Isolated Perfused Pig Liver

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Abstract
The metabolism of fructose andd‐glyceraldehyde by the isolated perfused pig liver and kinetics of relevant pig‐liver enzymes were studied. Fructose elimination by the isolated perfused organ could be described by Michaelis kinetics with an apparentKmof 5.9 mM and a maximum elimination rate of 2.9 μmol × g−1× min−1. The Kmfor pig liver ketohexokinase was 0.5 mM and the maximum activity 1.9 U/g at 25°C. Fructose 1‐phosphate up to 6.3 mM was measured in biopsies taken at the end of the experiments. The reversible inhibition of the aldolase cleavage of fructose 1‐phosphate was demonstrated by renewed formation ofd‐glyceraldehyde after an initial high dose of fructose had been eliminated. During steady‐state fructose elimination the concentration ofd‐glyceraldehyde in the medium was below 0.5 mM. The elimination kinetics ofd‐glyceraldehyde did not follow simple Michaelis kinetics, and the maximum elimination rate could not be measured. d‐glyceraldehyde was metabolized to glycerol, glycerate, and (probablyviatriokinase) to lactate. Net glycerol formation took place only at concentrations ofd‐glyceraldehyde above 1 mM. Glycerate accumulated even at low concentrations ofd‐glyceraldehyde. The findings are in accordance with the kinetics of the appropriate enzymes measuredin vitro. More glucose than lactate was formed from fructose, while the reverse was true ford‐glyceraldehyde. Lactate, pyruvate, glucose, glycerol, glycerate,d‐glyceraldehyde, sorbitol, malate, citrate, glutainate, and ketone bodies accounted for less than half the amount of fructose metabolized. The concentration of sorbitol, malate, and citrate rose during fructose metabolism. Fructose causes a small transient fall in the lactate to pyruvate concentration ratio. The oxygen consumption was increased 30% by fructose.

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