Formation of urocanic acid versus histamine from histidine in chub mackerel (Scomber japonicus) fillets as determined by a mixed-mode HPLC method

Urocanic acid, like histamine, is a degradation product of histidine that may accumulate over the storage of histidine-rich pelagic fish. Recently evidence from the dermatological and related fields has directed the attention of food scientists toward the adverse health effects of urocanic acid, mostly as a possible “missing factor” responsible for scombrotoxicosis besides histamine. However, experimental effort to study the hypothesis remains scarce, especially regarding the formation of urocanic acid in comparison to the well-documented histamine. Here we report a mixed-mode HPLC method for simultaneously analyzing urocanic acid (both the trans - and cis -isomers), histamine, and histidine in chub mackerel ( Scomber japonicus ) fillets and the use of the method to monitor their changes in concentration at typical refrigeration and abuse temperatures. The method features a simple, reliable chromatographic analysis in cation-exchange/reversed-phase/hydrophilic-interaction triple mode of retention, without the need of precolumn derivatization or addition of ion-pairing agents. At both 4 and 20 °C slow but consistent formation of trans -urocanic acid was observed over time with a relatively faster rate at 20 °C. Histamine formation, by contrast, occurred at 20 °C but ceased completely at 4 °C. In addition, we demonstrate the occurrence of trans -to- cis isomerization of UCA in chub mackerel fillets upon direct sunlight exposure: under our conditions, an isomerization yield around 13 % was achieved within hours on a sunny winter day. Overall, this study provides a chemical analysis method and relevant data for exploring the possible role of urocanic acid in the aetiology of scombrotoxicosis.