P450C11B3 MESSENGER-RNA, TRANSCRIBED FROM A 3RD P450C11 GENE, IS EXPRESSED IN A TISSUE-SPECIFIC, DEVELOPMENTALLY, AND HORMONALLY REGULATED FASHION IN THE RODENT ADRENAL AND ENCODES A PROTEIN WITH BOTH 11-HYDROXYLASE AND 18-HYDROXYLASE ACTIVITIES

The rat genome contains four P450c11 genes, One of these (CYP11B1) enc odes P450c11 beta, which is the steroid 11 beta-hydroxylase found sole ly in the adrenal zona fas ciculata/reticularis, and is responsible fo r the conversion of 11-deoxycorticosterone to corticosterone. A second P450c11 gene (CYP11B2) encodes P450c11AS, which is the aldosterone sy nthase found solely in the adrenal zona glomerulosa, P450c11AS has thr ee activities, 11 beta-hydroxylase, 18-hydroxylase, and 18-oxidase, an d is responsible for the conversion of 11-deoxycorticosterone to aldos terone. Recently, two more rat P450c11 genes, P450c11B3 and P450c11B4, were cloned, P450c11B4 appears to be a pseudogene, as two exons are r eplaced by unrelated DNA, P450c11B3 closely resembles P450c11 beta in mRNA and encoded amino acid sequences, predicting a protein of 498 ami no acids. However, the expression of this mRNA and protein have not be en demonstrated to date, We now demonstrate that this P450c11B3 mRNA i s expressed in the adrenal gland several days after birth and is not e xpressed during fetal development or in the adult rat adrenal. Like P4 50c11 beta mRNA, P450c11B3 mRNA is expressed in the zona fasciculata/r eticularis and not in the zona glomerulosa, However, the regulation of P450c11B3 mRNA expression is different from that of P450c11 beta mRNA , in that its abundance is decreased by ACTH in a sex dependent fashio n, Transfection of eukaryotic cells with a vector expressing P450c11B3 shows that this form of P450c11 can convert 11-deoxycorticosterone (D OG) to corticosterone and thus has the same enzymatic activity as P450 c11 beta. In addition, P450c11B3 can convert DOC to 18-OH DOC and cort icosterone to 18-OH corticosterone and thus has 18-hydroxylase activit y similar to P450c11AS, but it lacks detectable 18-oxidase activity, T hus, P450c11B3 catalyzes 11 beta- and 18-hydroxylation and thus has a spectrum of activities midway between P450c11 beta and P450c11AS.