Truffles are ectomycorrhizal fungi which have a great dependence on carbohy
drates supplied by their host plants. The catabolism of hexoses in the myco
biont is important for the production of energy, and the first enzyme in th
e hexose assimilation pathways is hexokinase. This study reports difference
s in the expression of this enzyme during the growth of Tuber borchii Vitta
d. mycelium (strain ATCC 96540). Three hexokinase activities (HKM1, HKM2 an
d HKM3) were isolated by anion-exchange chromatography and partially purifi
ed. HKM1 and HKM2 were present in the linear phase at 15-50 days of growth.
Two remarkable differences were found in the sugar-phosphorylating activit
y and stability of HKM1 and HKM2. HKM2 did not phosphorylate the fructose a
nd it was present in the chromatographic profile only when substrates such
as glucose, glucosamine or mannose were added to the extraction buffer. On
the contrary, HKM1 utilized also fructose and was detected under all the ex
perimental conditions used. HKM3 was the only molecular form observed after
70 days, when the fungus growth had reached a plateau. To our knowledge th
ese results represent the first evidence for the presence in T. borchii myc
elium of three distinct enzymatic forms of hexokinase which are differently
expressed during growth of the fungus.