Interaction of pyridoxal phosphate with the amino groups at the active site of 5- aminolevulinic acid dehydratase in maize

Pyridoxal 5′-phosphate strongly and reversibly inhibited maize leaf 5-amino levulinic acid dehydratase. The inhibition was linearly competitive with respect to the substrate 5-aminolevulinic acid at pH values between 7 to 9.0. Pyridoxal was also effective as an inhibitor of the enzyme but pyridoxamine phosphate was not inhibitory. The results suggest that pyridoxal 5′-phosphate may be interacting with the enzyme either close to or at the 5-aminolevulinic acid binding site. This conclusion was further corroborated by the detection of a Schiff base between the enzyme and the substrate, 5-aminolevulinic acid and by reduction of pyridoxal phosphate and substrate complexes with sodium borohydride