Fructose and d ‐Glyceraldehyde Metabolism in the Isolated Perfused Pig Liver

The metabolism of fructose and d ‐glyceraldehyde by the isolated perfused pig liver and kinetics of relevant pig‐liver enzymes were studied.1 Fructose elimination by the isolated perfused organ could be described by Michaelis kinetics with an apparent K m of 5.9 mM and a maximum elimination rate of 2.9 μmol × g −1 × min −1 . The K m for pig liver ketohexokinase was 0.5 mM and the maximum activity 1.9 U/g at 25°C. 2 Fructose 1‐phosphate up to 6.3 mM was measured in biopsies taken at the end of the experiments. 3 The reversible inhibition of the aldolase cleavage of fructose 1‐phosphate was demonstrated by renewed formation of d ‐glyceraldehyde after an initial high dose of fructose had been eliminated. During steady‐state fructose elimination the concentration of d ‐glyceraldehyde in the medium was below 0.5 mM. 4 The elimination kinetics of d ‐glyceraldehyde did not follow simple Michaelis kinetics, and the maximum elimination rate could not be measured. 5 d ‐glyceraldehyde was metabolized to glycerol, glycerate, and (probably via triokinase) to lactate. Net glycerol formation took place only at concentrations of d ‐glyceraldehyde above 1 mM. Glycerate accumulated even at low concentrations of d ‐glyceraldehyde. The findings are in accordance with the kinetics of the appropriate enzymes measured in vitro . 6 More glucose than lactate was formed from fructose, while the reverse was true for d ‐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. 7 Fructose causes a small transient fall in the lactate to pyruvate concentration ratio. 8 The oxygen consumption was increased 30% by fructose.