Malic enzyme of Fusarium oxysporum

Abstract

The levels of the NADP-specific malic enzyme in Fusarium oxysporum are controlled by both the carbon and nitrogen sources in which the cells are grown. The enzyme is not induced by malate; maximum activity is obtained when the mycelium is grown on sucrose or ethanol. With ammonium sulfate as the nitrogen source, one enzyme-activity peak is obtained by 40 h, while in the presence of potassium nitrate this peak is repressed, with a peak appearing at a later stage of growth. In ammonium nitrate, two enzyme-activity peaks are present, one corresponding to the peak present in ammonium sulfate and the other to the peak present in potassium nitrate grown cells.When the initial velocities are plotted against increasing malate concentration non-Michaelis–Menton kinetics are obtained. The double-reciprocal plots are biphasic and Rs values of 161 are obtained. Hill plots prepared from initial velocity data show that at low malate concentration, the slope of the line is 0.87, and it decreases to 0.45 at 1.32 × 10−3 M malate. With increasing malate concentration the slope increased to a value of 1.0. It appears that this type of kinetic behavior is typical of a system in which there is negative cooperativity with respect to ligand binding with concurrent progressive substrate activation.