The prodrug mycophenolate mofetil (MMF) is used clinically for prophylaxis of organ rejection in transplant patients. MMF is metabolized to the active metabolite mycophenolic acid (MPA). While proven useful in this setting, a significant fraction of patients receiving MMF chronically experience delayed-onset diarrhea, which limits the long term effectiveness of their treatment. MPA is eliminated primarily through glucuronidation by the action of UDP-glucuronosyltransferase (UGT) 1A enzymes within the liver and intestine. Glucuronides in the liver are excreted into the bile by the canalicular transporter multidrug resistance-associated protein 2 (MRP2) where they are subject to enterohepatic cycling and excretion through the urine as glucuronides. Glucuronidation results in the formation of the inactive phenolic glucuronide (MPAG) and the labile acyl glucuronide (acMPAG). While the formation of MPAG is the primary method of detoxification of MPA in vivo, studies have attempted to link formation of acyl glucuronides with adverse drug reactions (ADRs). While evidence has been inconclusive in directly linking acyl glucuronides with toxicity, some drugs forming acyl glucuronides have been removed from the marketplace due to ADRs. The overall hypothesis of this dissertation project is that variable glucuronidation formation and efflux within the liver and gastrointestinal tract results in differential MPAG formation rates and thus modulates MPA toxicity. The study of the relationships between metabolism and transport was aided by quantitative measurement of relevant enzymes in humans and animals. This methodology was used to establish assays to quantify precise differences in UGT1A enzymes between the tissues in rats and humans to understand the differences in metabolism and toxicity of MPA between species. This dissertation research examined the disposition of acMPAG and MPAG in relation to expression levels of Ugt1a enzymes and efflux transporters using targeted quantitative proteomics. By correlating glucuronide catalysis and efflux with absolute quantification of Ugts and transporters, we increased our understanding of relationships between glucuronide formation and disposition and UGT enzymes. This research helps explain some differences in metabolism and elimination observed between rats and humans administered MPA and these relationships may be applied to other xenobiotics with Phase II substrates.