Radioassay-Based Approach to Investigate Fate and Transformation of Conjugated and Free Estrogens in an Agricultural Soil

14C-Labelling of the natural steroid estrogens 17α-estradiol, 17β-estradiol, and estrone

The worldwide environmental occurrence of natural steroid estrogens has drawn increasing concerns. However, the fate of the estrogens, especially the α-isomer of estradiol, in the environmental matrices is still obscure. Using ¹⁴C-radioactively labelled forms of these estrogens can facilitate and is sometimes a prerequisite for studying their transformation and residual distribution in the environment, but the availability of labelled compounds (owing to commercially high prices or unavailable) hampers such studies. Here we developed simple and stable methods to synthesize ¹⁴C-labelled estradiol isomers and estrone using relatively low-priced [carboxyl-¹⁴C]-labelled sodium acetate as a precursor. The radiochemical syntheses started from an enol lactone, which was prepared from nandrolone by oxidation to open the A-ring followed by recyclization. Inversion of the 17β-hydroxyl group into its 17α-form was achieved via the Walden inversion using the Mitsunobu reaction. [3-¹⁴C]-17β-estradiol, [3-¹⁴C]-17α-estradiol, and [3-¹⁴C]-estrone were synthesized in five, six, and seven steps with an overall radiochemical yield of 17.4%, 16.2%, and 13.9%, respectively. The synthesized ¹⁴C-labelled compounds provide materials for studying the fate and behavior of estrogens in complex environmental matrixes and for further synthesis of their ¹⁴C-labelled sulfate and glucuronide conjugates.

Sorption and degradation of 17β-estradiol-17-sulfate in sterilized soil-water systems

To identify abiotic processes that govern the fate of a sulfate conjugated estrogen, 17β-estradiol-17-sulfate (E2-17S), soil batch experiments were conducted to investigate the dissipation, sorption, and degradation of radiolabeled E2-17S under sterilized conditions. The aqueous dissipation half-lives (DT50) for E2-17S ranged from 2.5 to 9.3h for the topsoil of high organic carbon (OC) content (1.29%), but E2-17S remained at ∼80% of applied dose in the low OC (0.26%) subsoil by 14 d. The non-linear sorption isotherms indicated limited sorption of E2-17S, and the concentration-dependent log KOC values were 2.20 and 2.45 for the topsoil and subsoil, respectively. Additionally, two types of hydroxyl E2-17S (OH-E2-17S and diOH-E2-17S) were found as major metabolites in the aqueous phase, which represented 9-25% and 6-7% of applied dose for the topsoil and subsoil at 14 d, respectively. Free estrogens, 17β-estradiol (E2) and estrone (E1), were detected from the sorbed phase of the soil-water systems.

Modeling coupled sorption and transformation of 17β-estradiol-17-sulfate in soil-water systems

Animal manure is the primary source of exogenous free estrogens in the environment, which are known endocrine-disrupting chemicals to disorder the reproduction system of organisms. Conjugated estrogens can act as precursors to free estrogens, which may increase the total estrogenicity in the environment. In this study, a comprehensive model was used to simultaneously simulate the coupled sorption and transformation of a sulfate estrogen conjugate, 17β-estradiol-17-sulfate (E2-17S), in various soil-water systems (non-sterile/sterile; topsoil/subsoil). The simulated processes included multiple transformation pathways (i.e. hydroxylation, hydrolysis, and oxidation) and mass transfer between the aqueous, reversibly sorbed, and irreversibly sorbed phases of all soils for E2-17S and its metabolites. The conceptual model was conceived based on a series of linear sorption and first-order transformation expressions. The model was inversely solved using finite difference to estimate process parameters. A global optimization method was applied for the inverse analysis along with variable model restrictions to estimate 36 parameters. The model provided a satisfactory simultaneous fit (R(2)adj=0.93 and d=0.87) of all the experimental data and reliable parameter estimates. This modeling study improved the understanding on fate and transport of estrogen conjugates under various soil-water conditions.

Dissipation and transformation of 17β-estradiol-17-sulfate in soil-water systems

In the environment, estrogen conjugates can be precursors to the endocrine-disrupting free estrogens, 17β-estradiol (E2) and estrone (E1). Compared to other estrogen conjugates, 17β-estradiol-17-sulfate (E2-17S) is detected at relatively high concentrations and frequencies in animal manure and surface runoff from fields receiving manure. To elucidate the lifecycle of manure-borne estrogens and their conjugates in the environment, the fate of radiolabelled E2-17S in agricultural soils was investigated using laboratory batch studies with soils of different organic carbon (OC) content (1.29% for topsoil versus 0.26% for subsoil). E2-17S was found relatively persistent in the aqueous phase throughout the duration of the 14 d experiment. The aqueous E2-17S persisted longer in the subsoil (half-lives (DT₅₀)=64-173 h) than the topsoil (DT₅₀=4.9-26 h), and the aqueous persistence of E2-17S depended on its initial concentration. The major transformation pathway was hydroxylation, yielding mono- and di-hydroxy-E2-17S (OH-E2-17S and diOH-E2-17S). Free estrogens, E2 and E1, were only observed in the sorbed phase of the soil at low concentrations (∼1% of applied dose), which demonstrated that deconjugation and subsequent oxidation had occurred. Although deconjugation was not a major pathway, E2-17S could be a precursor of free estrogens in the environment.