The purpose of this study was to compare the enzymatic kinetics and distribution of cytochrome P450 2D (CYP2D) among different rat brain subcellular fractions. Rat brains were used to prepare total membrane, crude mitochondrial, purified mitochondrial, and microsomal fractions, in addition to total homogenate. Michaelis-Menten kinetics of the brain CYP2D activity was estimated based on the conversion of dextromethorphan (DXM) to dextrorphan using UPLC-MS/MS. Protein levels of CYP2D and subcellular markers were determined by Western blot. Microsomal CYP2D exhibited high affinity and low capacity, compared with the mitochondrial CYP2D that had a much lower (∼50-fold) affinity but a higher (∼six-fold) capacity. The apparent CYP2D affinity and capacity of the crude mitochondria were in between those of the microsomes and purified mitochondria. Additionally, the CYP2D activity in the whole homogenate was much higher than that in the total membranes at higher DXM concentrations. A CYP2D immune-reactive band in the brain mitochondria appeared at a lower MW but had a much higher intensity than that in the microsomes. Mitochondrial brain CYP2D has a much higher capacity than its microsomal counterpart. Additionally, brain homogenate is more representative of the overall CYP2D activity than the widely-used total membrane fraction.
Keywords: CYP2D; Michaelis-Menten kinetics; dextromethorphan; dextromethorphan-O-demethylase activity; drug metabolism; microsomes; mitochondria; rat brain.