Oxygen dependence of dopamine-beta-hydoxylase activity and lactate metabolism in synaptosomes from rat brain

Dopamine-beta-hydroxylase activity can be assayed in vitro in suspensions of rat hypothalamic synaptosomes by following tritium release from [2-3H]dopamine in the presence of monoamine oxidase inhibitors. The activity is inhibited by diethyldithiocarbamate, fusaric acid, amphetamine, reserpine, and desipramine. It is not sensitive to addition of tyrosine at the concentration present in cerebrospinal fluid, to added fumarate or to a rat heart extract known to contain potent inhibitors of the isolated enzyme. The reaction in air follows simple saturation kinetics with respect to variation in dopamine concentration with Km of 0.075 microM. There is reversible inhibition of activity when it is assayed at low oxygen tension. The apparent oxygen affinity is such that the enzyme does not appear saturated with respect to oxygen even at arterial oxygen tensions. Other reactions assayed exhibit less sensitivity to hypoxia: production of lactic acid from glucose becomes stimulated at 10-12 mm Hg; and oxidation of lactic acid is not inhibited at oxygen tensions above 3-4 mm Hg. Comparison of these data with the distribution of oxygen tensions present in brain suggests a basis for classifying oxygen dependent neurochemical reactions.