Activity of human CYP26A1 towards six proluciferin probe substrates and their ester derivatives was monitored. These included three monofluorobenzyl ether isomers and three five-membered heterocycles. Overall, luciferin substrates with a free acid group gave higher activities than the ester compounds. Also, luciferin derivatives with six-ring structures were better metabolized than those with five-rings. The best substrates identified in this study are Luciferin 6′ 3-fluorobenzyl ether (Luciferin-3FBE) and its methyl ester (Luciferin-3FBEME). Taken together, we describe eleven new probe substrates for CYP26A1 and demonstrate for the first time that CYP26A1 does not only accept acid substrates but can also metabolize esters.

Cytochromes P450 (CYPs) and UDP-glucuronosyltransferases (UGTs) are the most important human drug metabolizing enzymes, but their mutual interactions are poorly understood. In this study, we recombinantly co-expressed of each one of the 19 human members of the UGT families 1 and 2 with either CYP2C9, CYP2D6, or CYP4Z1 in fission yeast. Using these strains, we monitored a total of 72 interactions: 57 cases where we tested the influence of UGT co-expression on CYP activity and 15 cases of the opposite approach. In the majority of cases (88%), UGT co-expression had a statistically significant (p < 0.05) effect on P450 activity (58% positive and 30% negative). Strong changes were observed in nine cases, including one case with an activity increase by a factor of 23 (CYP2C9 activity in the presence of UGT2A3) but also four cases with a complete loss of activity. When monitoring the effect of CYP co-expression on the activity of five UGTs, activity changes were generally not so pronounced and, if observed, always detrimental. UGT2B7 activity was not influenced by CYP co-expression, while the other UGTs were affected to varying degrees. These data suggest the notion that mutual influence of CYPs and UGTs on each other’s activity is a widespread phenomenon.