Modulation of the Catalytic Activity of Brain Succinic Semialdehyde Reductase by Reaction with Pyridoxal 5′‐Phosphate

An NADPH‐dependent succinic semialdehyde reductase from bovine brain was inactivated by pyridoxal 5′‐phosphate. Spectral evidence is presented to indicate that the inactivation proceeds through formation of a Schiff's base with amino groups of the enzyme. After sodium borohydride reduction of the inactivated enzyme, it was observed that 1 mol phosphopyridoxyl residue was incorporated/mol enzyme monomer. The coenzyme, NADPH, protected the enzyme against inactivation by pyridoxal 5′‐phosphate. After tryptic digestion of the enzyme modified with pyridoxal 5′‐phosphate in the presence and absence of NADPH followed by [3H]NaBH4 reduction, a radioactive peptide absorbing at 310 nm was isolated by reverse‐phase HPLC. The amino acid sequence of the peptide identified a portion of the pyridoxal‐5′‐phosphate–binding site as the region containing the sequence I‐L‐E‐N‐I‐Q‐V‐F‐X‐K, where X indicates that the phenylthiohydantoin amino acid could not be assigned. The missing residue, however, can be designated as a phosphopyridoxyl lysine as interpreted from the result of amino acid composition of the peptide. It is suggested that the catalytic function of succinic semialdehyde reductase is modulated by binding of pyridoxal 5′‐phosphate to a specific lysyl residue at or near the coenzyme‐binding site of the protein.