Fructose metabolism via the pentose cycle in tumoral islet cells

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Abstract
In tumoral islet cells (RINm5F line) the phosphorylation of d‐fructose is catalyzed by hexokinase rather than fructokinase. Fructose 6‐phosphate appears to be preferentially channelled into the pentose cycle, as suggested by a ratio of d‐[1‐14C]fructose/d‐[U‐14C]fructose oxidation close to 2.7, the failure to generate 14C‐labelled lactate from d‐[1‐14C]fructose and a poor metabolic response to menadione. When the islet cells are exposed to both d‐fructose and d‐glucose, however, the metabolism of the former hexose is dramatically modified, fructose 6‐phosphate being now formed at a lower rate and preferentially channelled into the glycolytic pathway. These findings illustrate the existence of regulatory steps in fructose catabolism located distally to its site of phosphorylation.