Effect of micelle fatty acid composition and 3,4,3′,4′-tetrachlorobiphenyl (TCB) exposure on intestinal [14C]-TCB bioavailability and biotransformation in channel catfish in situ preparations

Polychlorinated biphenyls are transferred in the diet along aquatic food chains. This study investigated the effect of dietary micelle composition and 3,4,3',4'-tetrachlorobiphenyl (TCB) exposure upon the subsequent systemic bioavailability and intestinal metabolism of [C-14]-TCB in a catfish in situ intestinal preparation. Initial in vitro experiments examined the solubility of [C-14]-TCB in micelles of varying fatty acid composition. Micelles composed of single fatty acids demonstrated greater [C-14]-TCB solubility with those fatty acids of longer chain length. Similarly, micelles of the long-chain fatty acid, linoleic acid, solubilized more [C-14]-TCB than mixed micelles formulated from equal amounts of myristic (14:0), palmitic (16:0), stearic (18:0), or linoleic (18:2) acids. Systemic bioavailability of [C-14]-TCB (60 mu M) from an in situ perfused intestinal preparation was 2.2-fold greater when delivered to the intestine in linoleic acid micelles as compared to the mixed micelle preparation. Catfish exposed in vivo to either 0.5 or 5.0 mg TCB/kg feed for 10 days resulted in a 45 to 47% decrease in the subsequent systemic bioavailability of [C-14]-TCB in the in situ intestinal preparation. Total intestinal cytochrome P450 content was not significantly affected by TCB preexposure. Immunodetectable CYP1A was found only in the 5.0 mg TCB/kg diet treatment. Corresponding intestinal aryl hydrocarbon hydroxylase (AHH) activities were 2.46 +/- 1.16, 2.43 +/- 1.58, and 11.35 +/- 10.25 pmol/min/mg protein for the control, 0.5, and 5 mg TCB/kg diet groups, respectively. [C-14]-TCB in the in situ preparation was metabolized to only a small degree upon a single pass through the intestinal mucosa of the catfish. High variability and low rates of metabolism precluded the association of the magnitude of metabolism with dietary TCB pretreatment. Analysis of tissue sample extracts demonstrated 4 minor peaks, 3 of which were tentatively identified by co-elution with standards as 2-OH-3,4,3',4'-TCB, 4-OH-3,5,3',4'-TCB, and 5-OH-3,4,3',4'-TCB. A fourth remains unidentified. Histological changes in the intestine such as thinning of the submucosa and increased numbers of goblet cells were evident at the 5.0 mg TCB/kg diet dose. These results suggest that TCB intestinal bioavailability may be linked to micelle composition as well as TCB exposure history. Furthermore, single pass intestinal metabolism appears to be a minor contributor to the biotransformational modification of dietary TCB.