## 3 Links between the Chemistry and Biochemistry and Toxicology of CTXs
### Distribution of C-CTXs in Fish  
Caribbean CTXs distribute unevenly across fish tissues, including the liver, blood, kidney, spleen, skeletal muscle, heart, gonads, gills, skin, and bones [91; 150]. This distribution among different tissues allows C-CTXs [151; 152] and P-CTXs [153; 89; 154] to be retained in different amounts as they are transferred through the food chain. Their structural stability limits liver metabolism, which would normally be expected to accelerate deprivation. Oxidation of P-CTXs enhances potency without appearing to dramatically accelerate deprivation, given that the most oxidised form typically dominates in fish flesh [75; 89]. Similar findings have been observed in _G. funebris_ and other fish species, where the liver contains the highest concentration of C-CTXs. Various Caribbean fishes have been found to accumulate 5-43-times higher levels of C-CTXs in the liver than in flesh [91]. A study on 55 amberjacks from the Canary Islands, tested using the neuroblastoma cell-based assay (CBA-N2a), showed 17-times higher concentrations of C-CTXs in the liver compared to the flesh [112]. A recent study confirmed the unequal distribution of CTXs using CBA-N2a in fish by testing different muscle, liver, and gonad samples from four amberjacks (_Seriola_ spp.) and four dusky groupers (_Epinephus marginatus_) captured in the Canary Islands [155]. The caudal muscle exhibited more CTX activity than the other fractions of the fillet, while the highest ciguatoxin concentrations were found in the liver and gonads, from fish with lower levels of toxicity in their flesh [155]. Indeed, some fish that exhibited toxicity in liver may have had non-toxic and edible flesh [113]. This was also observed for Caribbean carnivorous fish (_Mycteroperca venenosa_, _Caranx bartholomaei_, _Seriola rivoliana_, _Gymnothorax funebris_) with edible flesh, despite their livers containing ciguatoxins at significant levels [156]. Not surprisingly, given its hydrophobicity, P-CTXs bind to proteins in fish liver [91] and skeletal muscle [157]. As fish nerves respond similarly to those of mammals, attachment to proteins could protect fish from the lethal effects of ciguatoxins [158]. This kind of protection is reminiscent of that observed for tetrodotoxin-contaminated invertebrates [159; 160]. The amount of Pacific ciguatoxins has also been measured in fish blood [161], mouse blood [162; 163], and human blood [164].  
A study of fish liver and human P-450 enzymatic oxidation of P-CTXs by [PERSON] et al. [90] confirmed that enzymatic oxidation of ciguatoxins occurs in the liver. Different rates of deprivation, time from initial exposure, and species-specific differences in the ability of P-450 enzymes to generate C-CTX congeners can explain the different toxin profiles found in fish [19; 23; 38; 41; 42; 45; 46; 75; 89; 91; 118]. Two glucuronide products of C-CTX-1/-2 were recently identified by [PERSON] et al. using high-resolution tandem mass spectrometry [165]. Their chemical reduction experiments confirmed that the metabolites were comprised of four distinct glucuronide products, with the sugar attached at two separate sites on C-CTX-1/-2, and the hydroxyl group on carbon C56 was excluded as the conjugation site [165]. Glucuronidation is a novel biotransformation pathway that has not been reported for other related polyether phycotoxins, yet its occurrence across all the tested fish species suggests that it could be a prevalent and important detoxification mechanism in marine organisms. The absence of glucuronidation observed in rat and human microsomes suggests that alternate biotransformation pathways may be dominant in higher vertebrates [165].