Journal of Agricultural and Food Chemistry
ARTICLE
for the enzyme, and last, by acting as a reductant of o-quinone. In
the short term, the most effective mechanism for inhibiting
browning could be the reduction of o-quinones by THF, altho-
ugh the most long-lasting process would be the suicide inactiva-
tion of the enzyme.
’ ASSOCIATED CONTENT
S
Supporting Information. Kinetic analysis of the pH
b
effect on the enzymatic activity. This material is available free
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ34-868883963. E-mail: canovasf@um.es.
Funding Sources
This paper was partially supported by grants from the Ministerio
de Educaciꢀon y Ciencia (Madrid, Spain) Projects BIO2009-
12956, CTQ2009-13023, and SAF2009-12043-C02-01 and the
Fundaciꢀon Sꢀeneca (CARM, Murcia, Spain) Projects 08856/PI/
08, 08813/PI/08, and 08595/PI/08. Jose Luis Mu~noz-Mu~noz
and Francisco García-Molina hold two fellowships from the
Fundaciꢀon Caja Murcia (Murcia, Spain).
Figure 5. Effect of pH on the oxidation rate of THF by TYR. (b)
Representation of VD0,THHFF versus pH. The experimental conditions were
30 mM sodium acetate buffer at pH 2.2, 2.5, 2.75, 3.0, 3.2, 3.5, 4.0, 4.5,
and 5.0 and 30 mM sodium phosphate buffer at pH 5.5, 6.0, 6.5, 7.0, 7.5,
8.0, 8.5, and 9.2, 62.5 μM THF, 0.26 mM O2, and 400 nM TYR. (9)
o-TBQ
0,TBC
Effect of pH on the oxidation of TBC by TYR. Representation of V
versus pH with 2 mM TBC and 2.5 nM TYR. The buffers used were the
same as for tetrahydrofolate oxidase activity.
the pteridin ring. In step 2, the nitrogen of the ring, which is
a powerful nucleophile because C4a has a low value of δ4a
(Table 1) and, therefore, a high electron density, can carry out
a nucleophilic attack on the copper of the active site, binding
axially, while the hydrogen, which is bound to the nitrogen, is
withdrawn by base B. In step 3, OH of the substrate transfers a
proton to histidine, binding biaxially, before oxidation/reduction
processes governed by step 4 originate the form Ed. Because of
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Inhibition of Browning in the Presence of THF. Browning
and pigmentation may diminish in the presence of THF. Browning
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dx.doi.org/10.1021/jf1035433 |J. Agric. Food Chem. 2011, 59, 1383–1391