Cytochrome P450 CYP1A1 accumulates in the cytosol of kidney and brain and is activated by heme

Cytochrome P450 CYP1A1 is expressed in most tissues. In brain and kidney, its function remains unclear because its enzymatic activity is barely measurable. Here, we report on the localization of CYP1A1 in the cytosol of kidney and brain, as revealed by immunoblotting with anti-CYP1A1 antibodies and by 7-ethoxyresorufin deethylation (EROD). Hematin (8 microM) added in vitro to cytosol increased the EROD-activity 10-fold in brain olfactory bulb and 7-fold in kidney, presumably by reconstitution of apocytochrome. Succinylacetone, an inhibitor of heme biosynthesis, increased the ratio of cytosolic to microsomal EROD activity of transiently expressed CYP1A1 in COS-1 cells from 1:1 to nearly 6:1. This indicates a strong decrease of microsomal activity with increasing succinylacetone concentration. CYP1A1 activities correlated with CYP1A1 protein assessed by immunoblotting. We conclude that the availability of heme is a limiting factor of P450 function in extrahepatic tissue. Our data further suggest that reduced availability of heme limits the incorporation of P450s into brain endoplasmic reticulum. These observations are important when assessing the function of P450s in extrahepatic tissue.