Identification of Human Sulfotransferases Active towards Silymarin Flavonolignans and Taxifolin

Abstract

Natural phenolic compounds are known to be metabolized by phase II metabolic reactions. In this study, we examined the in vitro sulfation of the main constituents of silymarin, an herbal remedy produced from the fruits of the milk thistle. The study focused on major flavonolignan constituents, including silybin A, silybin B, isosilybin A, isosilybin B, silychristin, and silydianin, as well as the flavonoid taxifolin. Using ultra-high-performance liquid chromatography coupled with tandem mass spectrometry (UHPLC-MS), individual flavonolignans and taxifolin were found to be sulfated by human liver and human intestinal cytosols. Moreover, experiments with recombinant enzymes revealed that human sulfotransferases (SULTs) 1A1*1, 1A1*2, 1A2, 1A3, 1B1, 1C4, and 1E1 catalyzed the sulfation of all of the tested compounds, with the exception of silydianin, which was not sulfated by SULT1B1 and SULT1C4. The sulfation products detected were monosulfates, of which some of the major ones were identified as silybin A 20-O-sulfate, silybin B 20-O-sulfate, and isosilybin A 20-O-sulfate. Further, we also observed the sulfation of the tested compounds when they were tested in the silymarin mixture. Sulfates of flavonolignans and of taxifolin were produced by incubating silymarin with all of the above SULT enzymes, with human liver and intestinal cytosols, and also with human hepatocytes, even though the spectrum and amount of the sulfates varied among the metabolic models. Considering our results and the expression patterns of human sulfotransferases in metabolic tissues, we conclude that flavonolignans and taxifolin can potentially undergo both intestinal and hepatic sulfation, and that SULTs 1A1, 1A3, 1B1, and 1E1 could be involved in the biotransformation of the constituents of silymarin.

Chirality Matters: Biological Activity of Optically Pure Silybin and Its Congeners

“…A highly detailed analysis of sulfation of silymarin flavonolignan has been recently Vrba et al [115] using all the major pure flavonolignans. This study investigated the sulfation of individual silymarin flavonoids by the human liver and intestinal cytosols.…”

Systematic review of pharmacokinetics and potential pharmacokinetic interactions of flavonolignans from silymarin

“…While SULT1A1*1, 1A1*2, 1A2, and 1C4 exhibited a higher activity with silybin A, SULT1A3 was more active with silybin B. Similarly, SULTs 1B1 and 1C4 were more active with isosilybin A, while SULTs 1A2 and 1E1 preferred isosilybin B 165 . Metabolism did not yield identical diastereomeric metabolites—the methyl derivatives of isosilybin B (but not of isosilybin A) were hydroxylated 167 .…”

“…Recent experiments with pooled human intestinal cytosols demonstrated that at least one sulfated metabolite was formed with all tested flavonolignans (silybin A, silybin B, isosilybin A, isosilybin B, silychristin, and silydianin). The intestinal cytosol had even higher sulfation activity than the liver cytosol and the percentage of the conversion was between 0.4% and 4.9% for individual flavonolignans 165 . The real biological situation can be however different since the final intestinal concentrations of conjugates result from the interplay between conjugation and deconjugation (see below, Enterohepatic circulation).…”

“…Several different monosulfates were found after incubation of silymarin, silybin A, silybin B, isosilybin A, isosilybin B, silychristin, and silydianin with both the human liver cytosolic fraction and hepatocytes. Nine recombinant human sulfotransferases (SULTs) 1A1*1, 1A1*2, 1A2, 1A3, 1B1, 1C4, and 1E1 catalyzed their sulfation except for silydianin, which was not sulfated by SULT1B1 and SULT1C4 165 . Similarly, in isolated human hepatocytes, all glucuronidated flavonolignans except for silydianin also formed sulfates 167 .…”

Computational Analysis of Chemical Space of Natural Compounds Interacting with Sulfotransferases

“…The cytosolic SULTs include 13 enzymes that can be found in many tissues [11]. SULTs metabolize a wide variety of substrates, including endogenous compounds like steroids and polysaccharides, natural compounds [12,13] and drugs [8]. The molecular mechanisms involved in the substrate specificity of different SULT isoforms have been previously studied [14][15][16][17][18][19][20][21][22][23][24][25].…”