Cell type-specific storage of dopamine beta-monooxygenase

Expression of dopamine P-monooxygenase (DBM), the enzyme that converts dopa mine into norepinephrine, is limited to adrenal chromaffin cells and a smal l population of neurons. We studied DBM trafficking to regulated granules b y stably expressing rat DBM in AtT-20 corticotrope tumor cells, which conta in regulated granules, and in Chinese hamster ovary (CHO) cells, which lack regulated granules. The behavior of exogenous DBM in both cell lines was c ompared with endogenous DEM in adrenal chromaffin cells. CHO cells secreted active DBM, indicating that production of active enzyme does not require f eatures unique to neuroendocrine cells. Pulse-chase experiments indicated t hat early steps in DBM maturation followed a similar time course in AtT-20, CHO, and adrenal chromaffin cells. Use of a conformation-sensitive DBM ant iserum indicated that acquisition of a folded structure occurred with a sim ilar time course in all three cell types. Cell type-specific differences in DBM trafficking became apparent only when storage in granules was examined . As expected, DBM was stored in secretory granules in chromaffin cells; CH O cells failed to store DBM. Despite the fact that AtT-20 cells have regula ted granules, exogenous DBM was not stored in these granules. Thus storage of DBM in secretory granules requires cell type specific factors.