Abstract
Incubation of maize NADP-malic enzyme with tetranitromethane (TNM) resulted in a total loss of enzyme activity. The loss of enzyme activity was not observed at pH 6.3 but at pH 8.0. NADP-malic enzyme was inactivated to almost 90 % by incubation with an 80-fold molar excess of TNM for 5 min at 30 °C. The substrate malate or Mg2+ alone gave no protection, while NADP provided considerable protection. NADP in the presence of malate and Mg2+ totally protected the enzyme activity, suggesting that tyrosine residue may be located at or near the active site of maize NADP-malic enzyme. The spectral analysis of the modified enzyme indicated that modification of at least one tyrosine residue per subunit resulted in complete loss of the enzyme activity. The fluorescence study of unmodified and modified enzymes postulated that essential tyrosine residue at maize NADP-malic enzyme is possibly involved in malate binding.
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