CYP 2W1 Endogenous Substrates
779
an authentic 4-hydroxy atRA standard and with the known metabolites (depletion Km = 9.4 6 3.3 nM and Vmax = 11.3 6 4.3 pmoles min21
of atRA formed by CYP3A7. The kinetic studies indicate that CYP2W1 pmole P45021) (Lutz et al., 2009). Like CYP26A1, retinoic acid is a
oxidized atRA with Km = 5.6 6 0.6 mM and Vmax = 0.000085 6 tight-binding ligand for CYP2W1 (Ks = 58.5 nM). Retinoic acid is
0.00004 second21. CYP2W1 also oxidized retinol to 4-hydroxy- oxidized by CYP2W1 to 4-hydroxy atRA but less efficiently than by
retinol, and retinal to a mixture of products whose identities were not CYP26. The Vmax/Km for atRA oxidation by CYP26A1 is 2.0E7 M21 s21
,
established.
whereas the Vmax/Km for atRA oxidation by CYP2W1 was 15.2 M21 s21
.
Trace amounts of the 2-hydroxy metabolite were observed in the Considering the binding affinity and relatively low catalytic efficiency to
oxidation of 17b-estradiol by CYP2W1. CYP2W1 also slowly oxidized retinoic acid, CYP2W1 is more likely associated with regulation of retinoic
farnesol to an unidentified singly-hydroxylated product.
acid localization than depletion of this substrate.
The physiologic concentration of retinoic acid in human plasma was
In conclusion, in this study, we have identified five substrates of
reported to be 1–3 ng/ml (approximately 3 to 10 nM) (De Ruyter et al., CYP2W1, including retinoic acid, retinol, retinal, 17b-estradiol, and
1979). The CYP2W1 Ks for retinoic acid was 63.7 6 5.9 nM, and farnesol. The retinoids are tight-binding ligands for CYP2W1 with
CYP2W1 oxidized retinoic acid with Km = 5.6 6 0.6 mM. The Ks value nanomolar binding affinity, whereas the other three substrates only
was approximately 6- to 20-fold higher than the physiologic concen- have micromolar binding affinities. CYP2W1 was shown to catalyze
tration, whereas the Km value was more than 600-fold higher than the the metabolism of all five compounds, but the only physiologically
physiologic concentration; however, the unbound tumor retinoic acid relevant substrates are the retinoids, with retinoic acid being oxidized
concentration may be higher than the unbound plasma concentration. to the 4-hydroxy derivative. The results imply that CYP2W1 is likely to
Given that CYP2W1 is highly expressed in tumors, the metabolism of play a role in localized retinoid metabolism, and this function is related
retinoic acid by CYP2W1 may still be physiologically important. to its distinct tumor-specific expression pattern.
Alternatively, considering the relative lower catalytic efficiency (Vmax/Km =
15.2 M21 S21), retinoic acid might participate in a tumor regulation
pathway by acting as potent inhibitor for CYP2W1.
Authorship Contributions
Participated in research design: Zhao, Wan, Yang, Hammock, Ortiz de
The concentration of plasma 17b-estradiol has more variability,
depending on the age and time in the menstrual cycle in women.
Younger women tend to have higher 17b-estradiol levels, ranging from
50–500 pg/ml (approximately 0.1 to 1 nM), and postmenopausal
women have 17b-estradiol values of 10–20 pg/ml (0.02–0.04 nM)
(Santanam et al., 1998). Under physiologic conditions, arachidonic acid
is mostly present in an albumin-bound form. In the presence of albumin,
the concentration of free arachidonic acid (unbound) is typically below
0.1 mM (Brash, 2001). The metabolism of retinoic acid, 17b-estradiol,
and arachidonic acid have all been found to be related to tumor
development (Schneider et al., 1982; Marks et al., 2000; Tang and
Gudas, 2011); however, the physiologic concentrations of 17b-estradiol
and arachidonic acid are much lower than their dissociation constants
for binding to CYP2W1, which precludes a role for them as important
physiologic substrates for this enzyme. Retinoic acid thus appears to be
the only relevant physiologic substrate for CYP2W1 among a broad
range of potential endogenous substrates.
Montellano.
Conducted experiments: Zhao, Wan, Yang.
Performed data analysis: Zhao, Wan, Yang, Hammock, Ortiz de
Montellano.
Wrote or contributed to the writing of the manuscript: Zhao, Yang,
Hammock, Ortiz de Montellano.
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