Two allozymes (MDHf and MDHs) of cytoplasmic malate dehydrogenase of Drosophila virilis were partially purified and their biochemical properties were compared. MDHf has a pH optimum of 9.75 and MDHs one of 9.25 for malate oxidation. Optimal pH for oxaloacetate reduction is 6.75 and 8.0 for MDHf and MDHs, respectively. The Km value for oxaloacetate of MDHs is approximately twice as that of MDHf. No differences were found with respect to thermostability and Km's for malate, NAD+, or NADH. These results are discussed in terms of the physiological role of cytoplasmic malate dehydrogenase of D. virilis.
6 Figures and Tables
Fig. 2. Effect o fpH on the activity of cytoplasmic malate dehydrogenase allozymes. A: Malate oxidation. B: Oxaloacetate reduction. - - , 0.1 M phosphate buffer; - - - 0.1 M glycine-NaOH buffer, o, MDHf; o, MDH s.
Fig. 3. Relationship between the activities of cytoplasmic malate dehydrogenase allozymes and the concentration of malate and oxaloacetate, o, MDHf; o, MDH s.
Fig. 4. Lineweaver-Burk plots illustrating the effect of concentration ofcofactors on the activities of cytoplasmic malate dehydrogenase allozymes. A: NAD + reduction. B: NADH oxidation, o, MDHf; e, MDH s.
Fig. 5. Rates of thermal inactivation of cytoplasmic malate dehydrogenase allozymes at 45 C. Enzyme activity was measured with malate as substrate in 0.1 glycine-NaOH buffer, pH 9.5. o, MDHf; e, MDH s.
Table I. Purification of Cytoplasmic Malate Dehydrogenase from D. virilis a 437
Table II. Apparent Michaelis Constants for Cytoplasmic Malate Dehydrogenase Allozymes from D.
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