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Partial Purification and Characterization of Dihydrodipicolinic Acid Reductase from Maize ¹

Botany Department, University of Tennessee, Knoxville, Tennessee 37996-1100, Department of Medical Biology, University of Tennessee Memorial Research Center and Hospital, Knoxville, Tennessee 37920

Dihydrodipicolinic acid reductase, an enzyme which catalyzes the pyridine nucleotide-linked reduction of dihydrodipicolinic acid to tetrahydrodipicolinic acid in the biosynthetic pathway leading to L-lysine, has been partially purified from maize (Zea mays cv Pioneer 3145) kernels. The crude maize extract and the partially purified enzyme were assayed for dihydrodipicolinic acid reductase by their ability to restore the capability of crude extracts of a mutant Escherichia coli (CGSC 4549; defective in dihydrodipicolinic acid reductase) to synthesize diaminopimelic acid from aspartic acid and pyruvic acid.

In a study of its properties, the Michaelis constant obtained for dihydrodipicolinic acid was 4.3 x 10^–4 and for NADPH the K_m was 4.6 x 10^–5. The enzyme had a pH optimum close to 7 and was much more temperature labile than the bacterial enzyme. Its molecular weight was 8.0 x 10⁴.

Several compounds, viz., -picolinic acid, L-pipecolic acid, isophthalic acid, and isocinchomeronic acid, with structures similar to dihydrodipicolinic acid inhibited the reductase reaction. Dipicolinic acid, the most potent of these inhibitors, acted as a competitive inhibitor with a K_i of 9.0 x 10^–4. The competitive inhibition of the reductase reaction by dipicolinic acid (oxidized dihydrodipicolinic acid) suggests that the substrate for this enzyme was in the ring rather than the open chain form. Oxidized pyridine nucleotide inhibited the activity slightly.