Speaker
Description
Abstract:
Background: Cytochrome P450 (P450) enzymes are major catalysts involved in the metabolism of drugs, steroids, and fatty acids–hydrophobic molecules that interact with intracellular binding proteins in vivo. A focus is fatty acid binding protein 1 (FABP1), present in liver cytosol at ≥400 µM and reported to bind drugs tightly.
Methods: Purified recombinant human P450s and FABP1 were tested in steady-state and pre-steady-state kinetic assays. Kinetic modeling distinguished between mechanisms in which (i) P450 competes with FABP1 for binding in a “free equilibrium” and (ii) an alternative “direct delivery” in which FABP1 transfers substrates directly to a P450 for oxidation.
Results: FABP1 bound palmitic acid tightly and transferred it directly to P450 4A11 (mechanism ii above), clearly indicated by the lack of reaction inhibition. FABP1 bound diazepam and sulfinpyrazone with low-µM affinities and experiments with P450 3A4 also indicated direct substrate transfer from FABP1, but with varying degrees of reaction inhibition. Studies in microsomes and cytosol also favored direct transfer, although some attenuation of P450 catalytic activity is predicted at the high cellular concentration of FABP1.
Conclusions:
We provide evidence for the direct transfer of fatty acids and drugs from FABP1 to P450s 4A11 and 3A4. Binding to FABP1 may alter drug metabolism in a substrate-specific manner, and our work needs to be extended to more drugs and P450s. These findings have implications for pharmacokinetic modeling, in that FABP1 may alter the balance of tissue vs. plasma drug concentrations and the availability of drugs for P450s.
