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Soybean nodule xanthine dehydrogenase: a kinetic study.

M J Boland, D G Blevins, D D Randall

    Archives of Biochemistry and Biophysics
    |April 15, 1983
    PubMed
    Summary

    Soybean nodule xanthine dehydrogenase exhibits Ping-Pong kinetics and prefers xanthine oxidation. Its activity is pH-dependent, suggesting a catalytic role for a histidyl group, while thiol modification does not affect enzyme function.

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    Area of Science:

    • Biochemistry
    • Enzymology
    • Plant Science

    Background:

    • Xanthine dehydrogenase (XDH) is a critical enzyme in purine metabolism.
    • Understanding XDH kinetics and catalytic mechanisms is essential for plant biochemistry.

    Purpose of the Study:

    • To purify and characterize xanthine dehydrogenase from soybean nodules.
    • To elucidate the kinetic properties, substrate specificities, and catalytic mechanism of soybean XDH.

    Main Methods:

    • Enzyme purification from soybean nodules.
    • Kinetic analysis including Km, Vmax, and product inhibition studies.
    • pH dependence studies and chemical modification assays.

    Main Results:

    • Purified soybean XDH demonstrated Ping-Pong kinetic mechanism with xanthine and hypoxanthine as substrates.
    • Km values for xanthine, hypoxanthine, and NAD+ were determined.
    • pH dependence indicated a pKa of 7.6 for substrate oxidation and identified the protonated form of xanthine as the true substrate.
    • APAD+ was identified as an alternative electron acceptor.
    • Chemical modification suggested an essential histidyl group involved in catalysis.

    Conclusions:

    • Soybean XDH operates via a Ping-Pong mechanism.
    • The enzyme's catalytic activity is influenced by pH and likely involves a histidyl residue.
    • Kinetic data provides insights into purine metabolism in soybean nodules.

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