Hepatic in vitro biotransformation assays, in combination with in vitro-in vivo extrapolation (IVIVE) and bioaccumulation modeling, can be used to support regulatory bioaccumulation assessments. In most applications, however, these methods ignore the possibility of extrahepatic metabolism. Here we evaluated intestinal biotransformation in rainbow trout using S9 fractions prepared from the upper intestinal (GIT) epithelium. Measured levels of activity determined using standard substrates for phase I and phase II biotransformation enzymes were within 2-fold of activities measured in hepatic S9 fractions. In vitro intrinsic clearance rates for 2-ethylhexyl-4-methoxycinnamate (EHMC; an organic sunscreen agent) and two polycyclic aromatic hydrocarbons (pyrene [PYR] and benzo(a)pyrene [BAP]) were significantly higher in liver S9 fractions than in GIT S9 fractions. For octocrylene (OCT; a second sunscreen agent), however, in vitro intrinsic clearance rates were higher in GIT S9 fractions compared to liver S9 fractions. An existing ‘liver only’ IVIVE model was expanded to consider biotransformation in both the liver and GIT. Relevant IVIVE scaling factors were developed by morphological, histological, and biochemical evaluation of trout intestines. For chemicals biotransformed at higher rates by hepatic S9 fractions (i.e., BAP, PYR, EHMC), the ‘liver & GIT’ model yielded whole-body biotransformation rate constants (k_MET) that were within 1.2 to 1.4-fold of those estimated using the ‘liver only’ model. In contrast to these findings, the mean k_MET for OCT obtained using the ‘liver & GIT’ model was 3.3 times higher than the mean k_MET derived using the ‘liver only’ model and was in good agreement with empirical k_MET estimates determined previously for trout (<20 % difference). The results of this study suggest that current ‘liver only’ IVIVE approaches may underestimate in vivo biotransformation rates for chemicals that undergo substantial biotransformation in the GIT.

肝脏体外生物转化分析与体外-体内外推法 (IVIVE) 和生物蓄积模型相结合，可用于支持监管生物蓄积评估。然而，在大多数应用中，这些方法忽略了肝外代谢的可能性。在这里，我们使用从上肠 (GIT) 上皮细胞制备的 S9 级分评估了虹鳟鱼的肠道生物转化。使用 I 期和 II 期生物转化酶的标准底物测定的活性测量水平在肝脏 S9 级分中测量的活性的 2 倍以内。体外肝脏 S9 级分中 2-乙基己基-4-甲氧基肉桂酸酯（EHMC；一种有机防晒剂）和两种多环芳烃（芘 [PYR] 和苯并 (a) 芘 [BAP]）的内在清除率显着高于 GIT S9分数。然而，对于奥克立林（OCT；第二种防晒剂），与肝脏 S9 部分相比，GIT S9 部分的体外内在清除率更高。扩展了现有的“仅肝脏”IVIVE 模型以考虑肝脏和 GIT 中的生物转化。通过对鳟鱼肠道的形态学、组织学和生化评估，开发了相关的 IVIVE 缩放因子。对于通过肝脏 S9 组分（即 BAP、PYR、EHMC）以更高速率生物转化的化学品，“肝脏和 GIT”模型产生了全身生物转化速率常数（k _MET ) 是使用“仅肝脏”模型估计的值的 1.2 到 1.4 倍。与这些发现相反，使用“肝脏和 GIT”模型获得的 OCT平均k _MET比使用“仅肝脏”模型得出的平均k _MET高 3.3 倍，并且与先前确定的经验k _MET估计值非常一致鳟鱼（<20% 差异）。这项研究的结果表明，当前的“仅肝脏”IVIVE 方法可能低估了在GIT 中经历大量生物转化的化学物质的体内生物转化率。