Dopamine beta-hydroxylase: biochemistry and molecular biology.

Dopamine beta-hydroxylase (DBH) catalyzes the conversion of dopamine to norepinephrine (NE), and is known to exist in two forms: soluble and membrane-bound. It has been reported that the two forms are similar in their immunoreactivity, carbohydrate content, and binding affinities for various substrates, and are apparently dissimilar in subunit structures and hydrophilicity. Furthermore, added structural complexity is observed within sDBH itself. Our results indicate that purified sDBH, which runs a single band on a nondenaturing gel, exhibits three protein bands of 75 kDa, 72 kDa, and 69 kDa on SDS polyacrylamide gel. The majority of sDBH exists as a 72-kDa protein. The NH2-terminal amino acid sequence of this 72-kDa protein indicates that it consists of two polypeptides of equimolar concentrations, where one differs from the other by three extra amino acids at its NH2 terminus. Whether they are different proteolytic cleavage products is not known. Thus, the structure of DBH appears to be more complex than originally considered. In vitro translation of total mRNA of bovine adrenal medulla followed by immunoprecipitation of DBH produces a single 72-kDa band on SDS polyacrylamide gel. This suggests either that there is only one in vitro mRNA translation product, which is modified to become different forms of DBH, or that multiple translation products are present but are indistinguishable by molecular weight. These subjects have been discussed in detail in this paper.