ESTIMATING DE NOVO LIPOGENESIS in European Seabass (Dicentrarchus labrax L.) USING DEUTERATED WATER (2H2O)

Ivan Viegas*, João Rito, Miguel A. Pardal & John Jones
 
Center for Neuroscience and Cell Biology & Center for Functional Ecology, University of Coimbra, Portugal. UC-BIOTEC - BIOCANT, Parque Tecnológico de Cantanhede, Núcleo 4 - Lote 8, 3060-197 Cantanhede, Portugal
iviegas@ci.uc.pt

In aquaculture, there is high interest in substituting fishmeal protein with carbohydrate-based substrates such as vegetable starch. Furthermore, the provision of vegetable oil that reduces the dependence on marine-derived ingredients yields yet other significant environmental benefit for diet formulation. However it is still unclear how carnivorous fish assimilate these lipids, and whether these are metabolically significant and efficiently regulated. Farmed European seabass (Dicentrarchus labrax L.) have higher adiposity than their wild counterparts and this might be potentiated by carbohydrate feeding. Whether this reflects a reduction in fat oxidation, increased de novo lipogenesis (DNL), or both, is not known.

To study the effects of high carbohydrate feeding on hepatic triglyceride biosynthesis in European seabass (Dicentrarchus labrax L.), we measured triglyceride deuterium (2H)-enrichment following 6 days in deuterated water (2H2O) by 2H-NMR. Fish were fed with three isolipidic experimental diets: a standard fishmeal-based diet with no carbohydrates (CTRL), a diet with 30% raw starch (RS), and a diet with 30% digestible (gelatinized) starch (DS). The underlying principles of enrichment with 2H from 2H2O into other metabolites are well described and validated for mammals and even in fish in the case of alanine, glucose and glycogen. Due to poor sensitivity, 2H-NMR is dependent abundant samples, but compensates with refined positional enrichment information, besides giving a reliable composition profile.

While fractional synthetic rates (% day-1) were were close to insignificant in fish from every condition, glyceryl synthetic rates exceeded fatty acyl synthetic rates in all cases. DS and RS fish had significantly lower synthetic rates for glyceryl (~1% day-1) compared to CTRL. The same procedure was performed in extra-hepatic tissues (muscle and perivisceral fat) and this may provide an integrated analysis.

Our analysis indicated that elevated hepatic adiposity from digestible starch feeding was not attributable to increased de novo lipogenesis. This may not be the case in other tissues. Despite certain technical and methodological limitations, 2H2O has proven to be a reliable tool to yield a unique profile of de novo lipogenesis, fatty-acid chain synthesis and glycerol turnover activity. This approach can usefully complement lipidomic and gene-expression studies of tissue lipid origins and modifications in fish.