Abstract

There are wide individual variations in the thermal stability of human plasma dopamine-β-hydroxylase (DBH) during incubation at 55°. When the ratio of enzyme activity after heating at 55° for 20 minutes to that before heating (a heated-to-control, or H/C ratio) was used as a measure of thermostabiity, 10.2% of frozen serum samples from 362 randomly selected subjects had relatively thermolabile DBH (H/C < 0.86). These subjects also had a significantly lower average basal DBH activity (471 ± 49 units/ml, mean ± SEM, N = 37) than that of the randomly selected subjects with more thermostable enzyme (829 ± 25 units/ml, N = 325, p < 0.001). However, there was not a significant correlation of the trait of thermolabile DBH (H/C < 0.86) with the presence of the previously described allele for very low basal DBH enzymatic activity (< 50 units/ml). The results of experiments in which plasma from subjects with thermolabile and thermostable DBH were mixed and experiments in which DBH was partially purified from plasma by gel filtration chromatography were compatible with the conclusion that variations in the structural properties of DBH itself were probably related to variations in relative thermostability.