Rapid purification and radioimmunoassay of cytosolic malic enzyme

Redox regulation of maize NADP-malic enzyme by thiol-disulfide interchange: effect of reduced thioredoxin on activity

Incubation of C4 NADP-malic enzyme from maize leaves with the oxidant o-iodosobenzoate leads to the reversible and complete inactivation of the enzyme. The time-course of inactivation is biphasic with the rate depending on the o-iodosobenzoate concentration. The inactivation is partially prevented by L-malate, NADP and Mg2+ alone, while NADP plus Mg2+ afford total protection. The complete reversal of the inactivation by the reductive agents dithiothreitol and 2-mercaptoethanol suggests that the modification of the enzyme by o-iodosobenzoate occurs concomitant with the oxidation of one or more pairs of sulfhydryl groups to the disulfide state, producing a conformationally altered form of the protein or directly modifying the active site. Titration of free thiol groups before and after inactivation of maize malic enzyme by o-iodosobenzoate shows a decrease in the accessible groups from 7 to 5, suggesting inactivation is accompanied by oxidation of two vicinal thiols. The oxidized form of the enzyme is rapidly reactivated by incubation with chemical and photochemically reduced thioredoxin in vitro, while the 'dark' activity of the enzyme is enhanced to the level of the 'light' activity by dithiothreitol. This evidence suggests that a reversible reduction and oxidation of disulfide bonds may take place during the regulation of the enzyme, indicating that the redox state of the disulfide bonds of C4 NADP-malic enzyme from maize leaves is important for the expression of maximal catalytic activity.

Changes of malic enzyme activity in the developing rat brain are due to both the increase of mitochondrial protein content and the increase of specific activity

1. The pattern of NADP-linked malic enzyme activity estimated in the whole brain homogenate did not parallel that found in liver of developing rat. 2. Studies on intracellular distribution of malic enzyme in brain showed that the mitochondrial enzyme increased about three-fold between 10th and 40th day of life. Thereafter, a slow gradual increase to the adult level was observed. 3. The extramitochondrial malic enzyme from brain, like the liver enzyme, increased at the time of weaning, although to a lesser extent. At day 5 the brain malic enzyme was equally distributed between mitochondria and cytosol. 4. During the postnatal development, the contribution of the mitochondrial malic enzyme in the total activity was increasing, reaching the value approx. 80% at day 150 after birth. 5. The increase with age of the malic enzyme specific activity was observed in both synaptosomal and non-synaptosomal mitochondria, the changes in the last fraction being more pronounced. 6. The activity of citrate synthase developed markedly between 10-40 postnatal days, increasing about five-fold, while the specific activity of the enzyme did change neither in the synaptosomal nor in non-synaptosomal mitochondria at this period. 7. We conclude that the changes in malic enzyme activity in the developing rat brain are mainly due both to the increase of mitochondrial protein content and to the increase of specific activity of the mitochondrial malic enzyme.

NADP(+)-malic enzyme from sugarcane leaves: structural properties studied by thermal inactivation

The irreversible thermal inactivation of the sugarcane leaf NADP(+)-malic enzyme was studied at 50 degrees C and pH 7.0 and 8.0. Depending on the preincubation conditions, thermal inactivation followed mono- or biphasic first-order kinetics. A two-step behavior in the irreversible denaturation process was found when protein concentration was sufficiently low. The protein concentration necessary to obtain monlphasic thermal inactivation kinetics was lower at pH 8.0 than at pH 7.0. The results suggest that biphasic inactivation kinetics are the consequence of the existence of two different oligomeric forms of the enzyme (dimer and tetramer), with the dimer being more stable in regards to thermal inactivation. The effects of the substrate and essential cofactors on the thermostability and equilibrium between the dimeric and tetrameric enzyme forms were also studied. Depending on the pH, NADP+, L-malate, and Mg2+ all had a protective effect on the stability of the dimeric and tetrameric species during thermal treatment. However, these ligands showed different effects on the aggregation state of the enzyme. NADP+ and L-malate induced dissociation, especially at pH 8.0, whereas Mg2+ induced aggregation of the protein. By studying the thermal inactivation kinetics at 50 degrees C and different pH values it was observed that the equilibrium between dimers and tetramers was dramatically affected in the range of pH 7.0-8.0. These results suggest that an amino acid residue(s) in the protein with an apparent pKa value of 7.7 needs to be deprotonated to stabilize aggregation of the enzyme to the tetrameric form.

High malic enzyme activity in tumor cells and its cross-reaction with anti-pigeon liver malic enzyme serum

Rabbit antibodies against pigeon liver malic enzyme (EC 1.1.1.40) were prepared. The antiserum gave single precipitation line with crude pigeon liver extract. Cross reaction was observed with partially purified malic enzyme or crude extract from chicken liver. Positive cross reaction was also observed with the concentrated cytosolic fraction of two human carcinoma cell lines which were demonstrated to contain high malic enzyme activity. All other proteins examined did not react with the antibodies. When purified pigeon liver malic enzyme was mixed with the antiserum in vitro, a time-dependent inactivation of the enzyme activity was observed. Protection of the enzyme activity against antiserum inactivation was afforded by NADP+ or L-malate. Metal Mn2+ gave little protection.

Normal and diabetic malic enzyme: a biochemical comparison

The ability of insulin to influence the specific activity of hepatic malic enzyme was examined by comparing the biochemical properties of hepatic malic enzyme isolated from normal and diabetic rats. Even though these two enzyme species possess very different specific activities, they appeared biochemically identical upon two-dimensional gel electrophoresis, Western blot analysis, and in mixing experiments. The only major difference observed for the two enzyme molecules was in their respective Km and Vmax apparent values. These results appeared to indicate that the two enzyme molecules were structurally identical but functionally different. Suggestive evidence showed that this functional difference was the result of substrate inhibition of the diabetic malic enzyme molecule.

Affinity chromatography on 2',5'-ADP-Sepharose 4B for purification of malic enzyme from crustacean muscle

Shrimp abdomenal muscle NADP-dependent malic enzyme (E.C.1.1.1.40) was purified about 1500-fold to a specific activity of 48 units (mumol/min)/mg at 30 degrees C with good quantitative recovery in three chromatographic steps, including affinity chromatography on 2',5'-ADP-Sepharose 4B, a "substrate activation" method using malate substrate plus manganese chloride. In addition to the malate-manganese chloride substrate pair, succinate or glutamate plus manganese chloride or magnesium chloride could be used in this "substrate activation" method for crustacean NADP-malic enzyme purification on 2',5'-ADP-Sepharose 4B. Affinity chromatography alone purified malic enzyme almost 43 fold, and the overall method resulted in homogeneous enzyme since polyacrylamide gel electrophoresis of the native purified enzyme revealed only a single band staining for protein and enzyme activity.