Radioimmunoassay and Clearance of Circulating Dopamine-β-Hydroxylase

Abstract

Release of the enzyme dopamine-β-hydroxylase (DBH) into the circulation by activity of the sympathetic nervous system was demonstrated by enzymatic assay of serum DBH. The development of a sensitive and specific solid-phase radioimmunoassay suitable for routine assay of serum DBH is described. It is sensitive to 1 ng of purified sheep DBH and is reproducible with a coefficient of variation of 4%. The mean of serum values for five normal sheep was 0.59 ± 0.02 (SE) µg/ml. The assay was sufficiently sensitive to measure DBH in 10 µliters of human serum even though cross-reactivity with antibodies against sheep DBH was incomplete. Clearance of circulating DBH was measured by the decay of radioactivity due to ¹²⁵I-DBH after steady state intravenous infusion in lambs. The estimated half-life of circulating DBH after correction for labeled metabolites was approximately 3 hours, and it was cleared from blood at 250 ml/hour. Under resting conditions, the rate of release of DBH into the circulation was about 10 µg DBH/hour kg^-1 body weight. The distribution of radioactivity in various organs 7 to 48 hours after infusion showed that only the kidneys, lungs, and liver were significantly labeled. Since circulating DBH does not appear to be selectively taken up by sympathetic nerves nor rapidly metabolized, radioimmunoassay of serum DBH may be useful in measuring sympathetic activity in a wide range of physiological and disease states.