achieved on a C18 5U 150 mm 4.6 mm column, with a gradient run of the
mobile phase at 20–80% acetonitrile and water containing 0.1% trifluoroacetic
acid over 2 min, at a flow rate of 1 mL/min. Detection was performed at
370 nm.
Deconjugation is achieved by the use of the enzyme sulfatase, which
cleaves the sufonate group from the conjugate to release the parent compound.
Several types of sulfatases are commercially available, and the choice is usually
dictated by the multiple conjugation pathways involved. Thus, sulfatases with
or without glucuronidase activity can be utilized. A major limitation of
deconjugation is that the assay relies on quantitation of the parent
chemicalthe difference between the parent peak area at the start and end
of the sulfatase reaction gives an estimate of the product formed. This is not as
reliable as direct quantitation of the product itself against an authentic
standard. Another potential drawback is incomplete desulfation of the
product, leading to inaccurate estimation of sulfonate formation.
3.2.5.3 Liquid Chromatography–Mass Spectrometry (LC/MS) Sulfonate con-
jugates are easily detected by mass spectrometry with constant neutral loss
scans that are characteristic for the presence of sulfate (80 Da) conjugated
metabolites. Thus, several sulfonated conjugates of steroids and drugs have
been characterized with LC/MS techniques. Figure 3.7 depicts some examples
of sulfonated products detected by MS. Additional examples of LC/MS studies
to detect and quantify sulfonate conjugates are listed in Table 3.6.
3.2.6 SULT Inhibitors (Pacifici and Coughtrie, 2005)
One of the first SULT1A1 inhibitors identified in the rat liver was 2, 6-
dichloro-4-nitrophenol (DCNP) (Mulder and Scholtens, 1977). DCNP is a
dead-end inhibitor, and exhibits low IC 50 values toward SULT1A1 and
SULT1A3 (Seah and Wong, 1994). Hydroxylated polychlorinated biphenyls
(HPCBs) are potent inhibitors of recombinant human SULT1E1. HPCBs
exhibit low micromolar IC 50 values toward thyroid hormones (Schuur et al.,
1998). Several dietary chemicals such as quercetin, curcumin, and flavones are
known to inhibit SULTs. Some commonly used drugs that inhibit SULT1A1
and SULT1A3 activity include NSAIDs such as mefenamic acid, naproxen,
and salicylic acid.
3.2.7 Drug–Drug Interactions and Sulfonation
Drug–drug and food–drug interactions have been reported due to inhibition of
SULT activity in the human liver as well as the duodenum. Salbutamol
sulfonation was inhibited by salicylic acid more potently in the liver than the
68 CONJUGATIVE METABOLISM OF DRUGS