The Conversion of [4‐³H]Fructose and of [4‐³H]Glucose to Liver Glycogen in the Mouse

[4‐ 3 H,6‐ 14 C]Fructose or [4‐ 3 H,6‐ 14 C]glucose were injected into mice and the recovery of the two isotopes in liver glycogen was measured. As a mean, the 3 H/ 14 C ratio in glycogen was 36.6 and 51.8% of that in the injected fructose and glucose, respectively. From the comparison of these two ratios, the loss of tritium proper to fructose metabolism was equal to 29.2%. In the same experiments, the C‐6 to C‐1 randomization proper to fructose metabolism was, as a mean, equal to 27.4%. These findings are in agreement with the hypothesis that the glyceraldehyde formed in the liver upon aldolic splitting of fructose 1‐phosphate is metabolized by phosphorylation into glyceraldehyde 3‐phosphate under the action of triokinase. The oxidation of d ‐glyceraldehyde into d ‐glycerate would have resulted in a complete loss of tritium in glycogen. Furthermore, the 3 H/ 14 C ratio in d ‐glyceraldehyde formed from fructose was the same as in the precursor despite the fact that the aldehyde dehydrogenase discriminates against [1‐ 3 H]glyceraldehyde. Our results therefore preclude any important participation of the glycerate pathway in the metabolism of fructose. In the isolated rat diaphragm, the loss of tritium relative to 14 C was of the order of 20%, for both glucose and fructose, confirming that, in muscle these two sugars are converted to glycogen by the same pathway. When incubated in vitro with a liver preparation deprived of glucose 6‐phosphatase, doubly labelled UDPG lost no 3 H relative to 14 C. Doubly labelled hexose 6‐phosphates underwent a rapid loss of tritium in the same conditions, and this loss was cancelled by fructose diphosphate. It appears probable that the tritium loss is due to an interconversion of hexosephosphates with pentosephosphates, the hydrogen exchange occurring at the level of the pentosephaosphate 3‐epimerase. In agreement with an exchange of hydrogen at the level of hexose 6‐phosphates in the liver, it was observed that the decay of [4‐ 3 H]glucose in the blood is faster than that of [U 14 C]glucose.