Inhibition of human cytochrome P450 2E1 and 2A6 by aldehydes: Structure and activity relationships

The purpose of this study was to probe active site structure and dynamics of human cytochrome P450(2E1) and P450(2A6) using a series of related short chain fatty aldehydes. Binding efficiency of the aldehydes was monitored via their ability to inhibit the binding and activation of the probe substrates p-nitrophenol (2E1) and coumarin (2A6). Oxidation of the aldehydes was observed in reactions with individually expressed 2E1, but not 2A6, suggesting alternate binding modes. For saturated aldehydes the optimum chain length for inhibition of 2E1 was 9 carbons (K-I = 7.8 +/- 0.3 mu M), whereas for 2A6 heptanal was most potent (K-I = 15.8 +/- 1.1 mu M). A double bond in the 2-position of the aldehyde significantly decreased the observed KI relative to the corresponding saturated compound in most cases. A clear difference in the effect of the double bond was observed between the two isoforms. With 2E1, the double bond appeared to remove steric constraints on aldehyde binding with KI values for the 5-12 carbon compounds ranging between 2.6 +/- 0.1 mu M and 12.8 +/- 0.5 mu M, whereas steric effects remained the dominant factor in the binding of the unsaturated aldehydes to 2A6 (observed K-I values between 7.0 +/- 0.5 mu M and > 1000 mu M). The aldehyde function was essential for effective inhibition, as the corresponding carboxylic acids had very little effect on enzyme activity over the same range of concentrations, and branching at the 3-position of the aldehydes increased the corresponding K-I value in all cases examined. The results suggest that a conjugated pi-system may be a key structural determinant in the binding of these compounds to both enzymes, and may also be an important feature for the expansion of the active site volume in 2E1. (C) 2014 Elsevier Ireland Ltd. All rights reserved.