Expression of fructokinase isoforms in the sugarcane culm

Two isoforms of fructokinase (FRK), FRK1 and FRK2, are present in sugarcane (Saccharum spp. var N19) internodal tissue. Both isoforms are highly specific for fructose as the hexose substrate. FRK2 is inhibited by fructose concentrations exceeding 0.1 mM and 50% inhibition is attained at 230 μM (Ki (Fru) = 160 μM), while FRK1 activity is not negatively affected even at 1.0 mM fructose. The ratio of FRK2 to FRK1 activity is dependent on the developmental stage of the tissue. FRK1 appears to be the isoform that is preferentially expressed in mature tissue. Total FRK activity decreases during tissue maturation. This is the result of changes in expression of the isoforms and not inactivation of existing protein. A mathematical method that allows accurate estimation of the activities of the two isoforms of FRK in crude sugarcane extracts is presented.