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PLANT PHYSIOLOGY , Vol 108, Issue 2 581-588, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Assay, Purification, and Partial Characterization of Choline Monooxygenase from Spinach
M. Burnet, P. J. Lafontaine and A. D. Hanson
Horticultural Sciences Department, University of Florida, Gainesville, Florida 32611-0690
The osmoprotectant glycine betaine is synthesized via the path-way choline
-> betaine aldehyde -> glycine betaine. In spinach (Spinacia
oleracea), the first step is catalyzed by choline monooxygenase (CMO), and
the second is catalyzed by betaine aldehyde dehydrogenase. Because betaine
aldehyde is unstable and not easily detected, we developed a coupled
radiometric assay for CMO. [14C]Choline is used as substrate; NAD+ and
betaine aldehyde dehydrogenase prepared from Escherichia coli are added to
oxidize [14C]betaine aldehyde to [14C]glycine betaine, which is isolated by
ion exchange. The assay was used in the purification of CMO from leaves of
salinized spinach. The 10-step procedure included polyethylene glycol
precipitation, polyethyleneimine precipitation, hydrophobic interaction,
anion exchange on choline-Sepharose, dimethyldiethanolamine-Sepharose, and
Mono Q, hydroxyapatite, gel filtration, and sodium dodecyl
sulfate-polyacrylamide gel electrophoresis. Following gel filtration,
overall purification was about 600-fold and recovery of activity was 0.5%.
Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a
polypeptide with a molecular mass of 45 kD. Taken with the value of 98 kD
estimated for native CMO (R. Brouquisse, P. Weigel, D. Rhodes, C.F. Yocum,
A.D. Hanson [1989] Plant Physiol 90: 322-329), this indicates that CMO is a
homodimer. CMO preparations were red-brown, showed absorption maxima at 329
and 459 nm, and lost color upon dithionite addition, suggesting that CMO is
an iron-sulfur protein.
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