The effect of γ-radiation on the NADP-MALIC enzyme from mango fruit, mangifera indica-I. Physico-chemical aspects

1. 1. Malic enzyme purified from the fruit tissue of Mangifera indica was irradiated in dilute solution and the effect of γ-irradiation was investigated. 2. 2. The activity of the enzyme decreased exponentially as a function of the applied dose under all conditions investigated. The inactivation yield (Go-value) in neutral solution and in air was 0.069. 3. 3. The role of the radicals produced by water radiolysis in the inactivation of the enzyme was investigated by using different gas atmospheres and selective free radical-anions. The hydrogen atom and the hydrated electron (reducing species) were found to be important in the enzyme inactivation; as well as the possible destruction of cysteine and tryptophan residues. 4. 4. The irradiated enzyme appears to adopt a more compact conformation as reflected in a slightly lower M_r, Stokes-radius and diffusion coefficient. 5. 5. γ-Radiation does not lead to any heterogeneity in the charge and size properties of the enzyme and the pI and the M_r of the subunits were unaffected. 6. 6. Some differences in the amino acid composition of the non-irradiated and irradiated enzyme were observed but specific amino acid residues were not preferentially destroyed. 7. 7. These changes were also reflected in the ultraviolet spectrum of the enzyme which shifted to lower values. 8. 8. The major cause of inactivation seem to be a change in conformation caused by chemical modification of amino acid side chains.