Molecular Pharmacology, Vol 19, 444-450, Copyright © 1981 by the American Society for Pharmacology and Experimental Therapeutics

Human Dopamine--Hydroxylase

Comparison of the Enzyme from Plasma, Adrenal Medulla, and Pheochromocytoma by Radioimmunoassay

¹ Departments of Medicine, Veterans Administration Medical Center and the University of California, San Diego, California 92161

A homologous radioimmunoassay for human plasma dopamine--hydroxylase (DBH) is described. An antiserum specific for the purified plasma enzyme was found to cross-react fully with soluble DBH isolated from human pheochromocytoma and normal adrenal medulla. Similarly, an assay developed using pheochromocytoma DBH as an antigen gave equivalent results, and the two methods were interchangeable for the determination of immunoreactive DBH in human plasma. Bovine adrenal medullary DBH, on the other hand, cross-reacted poorly in these assays. The homospecific activity of DBH in pooled, normal human plasma, expressed as units of enzymatic activity per milligram of immunoreactive protein, ranged from 3.1 to 5.8 units/mg. This compares favorably with DBH in a chromaffin granule lysate from pheochromocytoma, 3.8 units/mg, as well as normal human adrenal medulla, 4.1 units/mg. Plasma DBH was further separated into enzymatically active tetrameric and dimeric fractions. The homospecific activity of the predominant tetrameric fraction from pooled plasma was about 20% greater than the dimeric enzyme. The results of this study are compatible with the conclusion that various forms of soluble human DBH are immunologically indistinguishable by homologous radioimmunoassay. Comparison of the enzymatic and immunoreactive levels of circulating DBH, on the other hand, may be altered by structural differences between the multiple forms of the plasma enzyme.

Note:
ACKNOWLEDGMENTS The authors wish to thank Dr. Richard A. Stone for the inspiration to initiate this study, Ms. Marta Zekan for her unexcelled typing skills, Michael Alford for assistance in computerizing the data analysis, and Annie Chen for technical assistance.