There is an increasing demand for glass eels in Japanese cuisine, but when eels are grown commercially at a high density, they have a tendency to change sex from female to male – likely an evolutionary adaptation for population control. Not a problem in itself, perhaps, but since male eels don’t tend to “measure up” (only female eels reach required market size), reversal of this natural masculinization is required. Sex-reversal can be induced by adding estradiol – a naturally occurring hormone – to eel feed; however, to ensure commercial growth, to meet regulatory approval from the FDA, and to guarantee consumer safety, levels of estradiol and its metabolites (estrone and estriol) in fish tissue must be monitored. To that end, aquaculture company NovaEel (based in Nova Scotia, Canada) partnered with researchers at Dalhousie University to develop an analytical platform that is up to the task.

Extracts of eel muscle were analyzed using reverse phase liquid chromatography coupled to differential ion mobility spectrometry/tandem mass spectrometry (LC-DMS-MS/MS) using a heated-assisted electrospray source. “In our lab, we’ve already been successful using this platform for proteomics and metabolomics workflows, so it was just a matter of tweaking the instrument’s LC and MS/MS acquisition parameters to produce satisfactory signals for the steroid hormones,” says Alejandro Cohen, Scientific Director of the Proteomics Core Facility at Dalhousie University. “After a week or two of fine tuning our methods, we achieved sub-ng detection limits per gram of tissue.”

DMS plays a key role in the platform by acting as an orthogonal pre-MS/MS separation mechanism and thus increasing the specificity of the method – something Cohen recommends when dealing with complex biological samples with ‘noisy’ backgrounds. But there can be a cost: “Optimal separation conditions for DMS does compromise the signal intensity somewhat,” he says. “However, I suggest users evaluate carefully to what extent the selectivity of DMS affects the sensitivity of their assays.”

Getting back to the eels... Estradiol was rapidly metabolized (50 percent depletion rate per day) and decreased to levels found in wild mature female eels (and the non-treated controls) five days after hormone treatment ended.

So feel free to re-join the line for the “eel-you-can-eat” buffet...