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PLANT PHYSIOLOGY , Vol 114, Issue 4 1237-1245, Copyright © 1997 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Purification and Characterization of a Novel Class III Peroxidase Isoenzyme from Tea Leaves
M. Kvaratskhelia, C. Winkel and RNF. Thorneley
Nitrogen Fixation Laboratory, John Innes Centre, Norwich, United Kingdom NR4 7UH (M.K., R.N.F.T.)
A novel, basic (isoelectric point > 10), heme peroxidase isoenzyme (TP;
relative molecular weight = 34,660 [plus or minus] 10, mean [plus or minus]
SE) that can account for a significant part of the ascorbate peroxidase
activity in tea (Camellia sinensis) leaves has been purified to
homogeneity. The ultraviolet/visible absorption spectrum is typical of
heme-containing plant peroxidases, with a Soret peak at 406 nm ([epsilon] =
115 mM-1 cm-1) and an A406/A280 value of 3.4. The enzyme has a high
specific activity for ascorbate oxidation (151 [mu]mol min-1 mg-1), with a
pH optimum in the range of 4.5 to 5.0. Substrate-specificity studies have
revealed significant differences between TP and other class III
peroxidases, as well as similarities with class I ascorbate peroxidases.
TP, like ascorbate peroxidase, exhibits a preference for ascorbate over
guaiacol, whereas other class III isoenzymes are characterized by
2-orders-of-magnitude higher activity for guaiacol than for ascorbate. TP
also forms an unstable porphyrin [pi] cation radical-type compound I, which
is converted to compound II within approximately 2 min in the absence of
added reductant. Amino acid sequence data show TP to be the first example,
to our knowledge, of a class III peroxidase with a high specificity for
ascorbate as an electron donor.
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