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Dissociation of changes in enzymatic and immunoreactive rat serum dopamine bets-hydroxylase during growth and
Abstract:
Dopamine beta-hydroxylase (DBH) activity was measured in the serum, heart and salivary glands of Sprague-Dawley rats from 1 day after birth to 90 days of age. Serum DBH activity in blood from newborn animals was 90 units, approximately 4.5 times higher than in blood from 60- to 90-day-old rats. The serum enzyme activity increased to 130 units at 15 days of age and then decreased rapidly to adult levels (20 units). This decrease was not due to changes in levels of circulating inhibitors or activators of the enzyme. Four different inbred strains of rats also demonstrated a striking decrease in serum DBH activity between 15 and 60 days of age. Cardiac DBH in Sprague-Dawley rats increased approximately 5-fold from birth to 15 days of age, but failed to decrease thereafter. Salivary gland DBH activity also increased with maturation and failed to decrease. Plasma concentrations of epinephrine and norepinephrine were no different in blood samples obtained from Sprague-Dawley rats 15 and 60 days of age. This finding makes it less likely that the decrease in serum DBH activity with maturation represents a decrease in the functional activity of sympathetic nerve terminals. Antibodies against DBH were used to measure immunoreactive DBH protein in serum from rats 15 and 60 days of age. These studies demonstrated a significantly higher enzymatic activity per unit of immunoreactive protein in blood from young rats than in blood from animals 60 days of age. The ratios of enzymatic activity to immunoreactive protein as measured by the quantity of antibody necessary to precipitate 50% of the endogenous serum DBH activity (AD50) were 21.18 +/- 1.04 and 3.83 +/- 0.40 (mean +/- S.E.M.) for blood from animals 15 and 60 days of age, respectively. Among the possible explanations for this observation are included a greater quantity of enzymatically inactive but immunoreactive DBH in blood of adult animals as compared with the blood of young rats, or the existence of a different form of serum DBH in blood of young animals.