Inheritance plays an important role in the determination of human plasma dopamine-β hydroxylase (DBH) enzymatic activity. It has been demonstrated that an allele (d) for very low enzymatic plasma DBH is inherited as an autosomal recessive trait. A radioimmunoassay for human DBH was developed to test the hypothesis that the presence of this allele results in a decrease in plasma DBH protein levels. The mean immunoreactive DBH (IDBH) in blood from a randomly selected population of adolescents was 824 ± 38 ng/ml (mean ±SEM, n=134). The correlation coefficient of enzymatic DBH with IDBH for this group of 134 adolescents was 0.84 (P<0.001). Of these subjects, 3.7% had values of <100 ng/ml and appeared to compose a separate subgroup analogous to the 3-4% of the population that is homozygous for the allele for low enzymatic activity. There was a significant sibling-sibging correlation of IDBH values in the 14 sibling pairs included among the 134 subjects studied (r=0.60, P<0.025). IDBH was also measured in blood from 56 subjects homozygous (dd) for the allele for low enzymatic DBH (enzymatic activity <50 U/ml) and in blood of 80 first-degree relatives of homozygous probands. All but two dd subjects had IDBH levels of <100 ng/ml. Results of family studies were compatible with the autosomal recessive inheritance of an allel for IDBH levels of less than 100 ng/ml which segregates with the allele for very low enzymatic activity. Average IDBH in blood of 37 obligate heterozygotes as determined by family studies (Dd) was 599±53 ng/ml (mean ±SEM), significantly lower than the IDBH values found in a randomly selected population (P<0.005). These results are compatible with the conclusion that the presence of the allele for low plasma enzymatic DBH results in a decrease in the quantity of DBH protein in humanplasma.
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