A NOVEL CYTOCHROME-P450 EXPRESSED PRIMARILY IN BRAIN

hct-1 (hippocampal transcript) was detected in a differential screen o f a rat hippocampal cDNA library. Expression of hct-1 was enriched in the formation but was also detected in rat liver and kidney, though at much lower levels; expression was barely detectable in testis, ovary, and adrenal. In liver, unlike brain, expression was sexually dimorphi c; hepatic expression was greatly reduced in female rats. In mouse, br ain expression was widespread, with the highest levels being detected in corpus callosum; only low levels were detected in liver. Sequence a nalysis of rat and mouse hct-1 cDNAs revealed extensive homologies wit h cytochrome P450s (CYPs), a diverse family of heme-binding monooxygen ases that metabolize a range of substrates including steroids, fatty a cids, and xenobiotics. Among the CYPs, hct-1 is most similar (39% at t he amino acid sequence) to cholesterol 7 alpha-hydroxylase (CYP7) and contains a postulated steroidogenic domain present in other steroid-me tabolizing CYPs but clearly represents a type of CYP not previously re ported. Genomic Southern analysis suggests that a single gene correspo nding to hct-1 is present in mouse, rat, and human. hct-1 is unusual i n that, unlike all other CYPs described, the primary site of expressio n is in the brain. Similarity to CYP7 and other steroid-metabolizing C YPs may argue that hct-1 (CYP7B) plays a role in steroid metabolism in brain, notable because of the documented ability of brain-derived ste roids (neurosteroids) to modulate cognitive function in vivo.