title: Reversible modification of pig heart mitochondrial malate dehydrogenase by pyridoxal 5'-phosphate.
abstract: 1. Pig heart mitochondrial malate dehydrogenase incubated with pyridoxal 5'-phosphate at pH 8.0 and 25 degrees C gradually loses activity. Such inactivation can be largely reversed by dialysis or by addition of L-lysine or L-cysteine, and can be made permanent by NaBH4 reduction. 2. Modification of malate dehydrogenase with pyridoxal 5'-phosphate at 35 degrees C involves two phases, an initial inactivation which is reversible and a slower irreversible second stage. 3. The initial reaction between pyridoxal 5'-phosphate and malate dehydrogenase appears to involve reversible formation of a Schiff base with the epsilon-amino group of a lysine residue. 4. Inactivation of malate dehydrogenase by pyridoxal 5'-phosphate at 10 degrees C involves only the reversible reaction. 5. At 10 degrees C repeated cycles of treatment with pyridoxal 5'-phosphate and NaBH4 reduction lead to a stepwise decline in residual activity. 6. Apparent Km values for malate and NAD+ are unaltered in the partially inactivated enzyme. 7. NAD+ and NADH give only partial protection against pyridoxal 5'-phosphate inactivation. Substrates give no effect.
mesh_terms: D001894:Borohydrides; D003545:Cysteine / Q000378:metabolism; D007700:Kinetics; D008291:Malate Dehydrogenase / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008293:Malates / Q000378:metabolism; D008931:Mitochondria, Muscle / Q000201:enzymology*; D009206:Myocardium / Q000201:enzymology / Q000648:ultrastructure; D009243:NAD / Q000378:metabolism; D011732:Pyridoxal Phosphate / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013696:Temperature; D013997:Time Factors
chemical_list: D001894:Borohydrides; D008293:Malates; D009243:NAD; D011732:Pyridoxal Phosphate; D008291:Malate Dehydrogenase; D003545:Cysteine