Branched-chain fatty acids in 7-O-acyl okadaite esters from bivalves

The okadaites, a group of polyether compounds involved in the human syndrome diarrhetic shellfish poisoning (DSP), include three major parent toxins (okadaic acid and the analogues DTX1 and DTX2) which in bivalves are mostly esterified at C7 with fatty acids (FA). Most of these esters include common saturated and unsaturated FA. Others involve odd FA and multi-branched FA. The major unidentified 7-O-acyl esters with the okadaites in Portuguese bivalves were confirmed here, by high-resolution mass spectrometry (HRMS), to present a side chain with the same exact mass as the OA and DTX2 palmitoyl esters (isopC16:0). A common chain with this exact mass, and abundant in the marine environment, is 4,8,12-trimethyltridecanoic acid (TMTD), which is a chlorophyll degradation product. Although all bivalves assayed contain the isopC16:0 ester, the unknown peaks were more abundant in sand-dwelling bivalves (clams, cockles) than in suspension-feeding bivalves (mussels). In addition to the high abundance of isopC16:0 combined with OA and DTX2 in natural samples, the isopC16:0 ester was also obtained by feeding bivalves with Prorocentrum lima containing DTX1. If this metabolization is attributable to the bivalves themselves, to a particular bacterial flora in sand-dwelling bivalves, or both, is a question that remains open.