Juliana Camacho Pereira,  Laudiene E.Meyer¹, Antonio Galina ^1¶

Hexokinase (HK) catalyses the crucial step in hexose metabolism converting free hexose and ATP to hexose-6-phosphate and ADP. The existence of particulate HK isoforms has been observed in plants. A substantial portion of hexokinase activity in plants is associated with outer mitochondria membrane (mt-HK) and plastids (plast-HK). Previous studies have shown that some mt-HK activities are inhibited by micromolar concentrations of ADP. The aim of this work was to evaluate the kinetic properties of mt-HK and plast-HK, using mitochondria and plastid isolated with Percoll gradient from potato tubers. Both mt-HK and plast-HK activities were inhibited by ADP at milimolar range using either glucose or fructose as substrates. The mt-HK and plast-HK have similar affinities for hexoses (Glc K_m = 0.05 to 0.11 mM and Frut K_m = 1.4 to 2.1 mM). Curiously, the inhibition of mt-HK by ADP is differently modulated depending of the sugar substrate used. The mt-HK has very low inhibition by ADP when the sugar substrate is fructose. When glucose is phosphorylated, a much more stringent regulation by ADP is observed (IC₅₀ = 100 to 200 mM). This pattern of ADP regulation is not found in plast-HK, where ADP is a strong inhibitor of the reaction independently of the sugar phosphorylated (IC₅₀ = 10 to 20 mM).