POTATO (SOLANUM-TUBEROSUM L.) FRUCTOKINASE EXPRESSED IN YEAST EXHIBITS INHIBITION BY FRUCTOSE OF BOTH IN-VITRO ENZYME-ACTIVITY AND RATE OF CELL-PROLIFERATION

A cDNA clone of potato (Solanum tuberosum) fructokinase (EC 2.7.1.4) w as expressed in mutant yeast cells unable to phosphorylate, or grow on either glucose or fructose. The transformed yeast cells complemented growth on fructose but not on glucose. Crude protein extracts of the t ransformed yeast showed phosphorylation activity with fructose as subs trate, but not with glucose. In addition, in vitro enzyme activity was substrate inhibited at fructose concentrations above 2 mM. Proliferat ion of the transformed yeast cells was also inhibited by increasing fr uctose concentrations, indicating that the phenomenon of substrate inh ibition may occur in vivo. These data suggest that substrate inhibitio n of fructokinase in potato tubers is a physiologically significant ph enomenon. (C) 1997 Elsevier Science Ireland Ltd.