Constitutive Forms of Drug Metabolizing P450s in the Brain

Identification of the drug metabolizing P450s in the brain is even more complicated than identification of the steroidogenic forms. Some of the reasons for this are: the large number of members in each subfamily; the overlapping substrate specificities even in members belonging to distinct subfamilies; cross reactivity of antibodies; and the fact that novel members of known families are constantly being discovered. There is, in addition, further complication of the issue by data from one laboratory, which reports very much higher levels of P450 and hepatic microsomal forms of P450 in the brain. These data, which are very different from those found in other laboratories, have been analyzed in a recent review (8) and will not be included in the present discussion. As is evident from Table 1, many members of family 1-4 have been detected by RT-PCR in the brain. Very few fulfill the criteria qualifying them as P450s of physiological or pharmacological relevance in the brain. Although there are many published studies on P450 catalytic activities in the brain, the overall picture of the level of hepatic P450s in the brain can best be illustrated by a very careful study by Jayyosi et al. published in 1992 (62). This lab has a long reputation in analyzing P450-catalyzed steroid metabolites and using testosterone as a model substrate for characterization of hepatic P450s (63). The position of hydroxylation on the testosterone molecule can be used to identify the form of P450 involved. In Table 3, data from the Jayyosi et al. paper has been used to calculate the contribution of individual forms of P450 to the P450 content of brain microsomes.

There are two main messages to be drawn from this table:

1. The brain has a very low capacity to hydroxylate steroids. This conclusion can be made because the same P450s that catalyze hydroxylations of testosterone also utilize estradiol, progesterone, glucocorticoids, and DHEA.

2. The forms of P450 in the table represent a very small fraction of the P450 in the brain. The brain P450 content is 3-5 pmol/mg microsomal protein. This means that the highest testosterone hydroxylase activity, i.e., production of 2p hydroxy testosterone can only account for 10% of the P450 and forms like 2B1 and 2A2 less than 0.5%. Similar calculations can be made for P450s of family 1A (64), 2D (67), 2E (68), and 4A (23). These are all families whose members have been detected by RT-PCR in the brain.

P450 4A3 is one of the few identified constitutive forms of P450 in the brain. It has been identified in the brain by the presence of its mRNA (RT-PCR) and protein (Western blotting and N-terminal sequencing) (23,41). Available antibodies were not suitable for immunohistochemical localization so the cellular distribution is not yet known. Members of the 4A subfamily are of physiological interest because they terminate the activity of many active metabolites of arachidonic acid (69).

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