Synthesis of omega-3 long-chain polyunsaturated fatty acid-rich triacylglycerols in an endemic goby, Gymnogobius isaza, from Lake Biwa, Japan.

It is commonly observed that freshwater fish contain lower amounts of omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs), such as eicosapentaenoic acid (EPA, C20:5n-3) and docosahexaenoic acid (DHA, C22:6n-3), than marine fish species. In this study, we performed a detailed comparative analysis of phospholipids (PLs) and triacylglycerols (TAGs) from Gymnogobius isaza, a freshwater goby endemic to Lake Biwa inhabiting the lake bottom, and Gymnogobius urotaenia, a related goby that inhabits the shore of Lake Biwa. We found that tissues from G. isaza contain remarkably high amounts of omega-3 LC-PUFAs in both PLs and TAGs. Mass spectrometry analysis of TAGs demonstrated that the most abundant TAG molecular species were TAG (16:0/18:1/20:5), followed by TAG (14:0/18:1/20:5), in which EPA is incorporated into TAG at either the sn-1 or sn-3 positions. We isolated cDNAs encoding acyl-CoA: diacylglycerol acyltransferase designated as GiDGAT1 and GiDGAT2, from G. isaza. Expression studies using a neutral lipid-deficient Saccharomyces cerevisiae mutant strain demonstrated that both GiDGAT1 and GiDGAT2 possessed diacylglycerol acyltransferase activity, and preferential incorporation of LC-PUFA into TAG was observed in the presence of GiDGAT1. This study revealed the novel lipid profiles of G. isaza and identified the enzymes that were involved in the production of PUFA-containing TAGs.