Abstract
Dopamine β-hydroxylase (EC 1.14.2.1.) activity showed a bimodal distribution pattern on isopycnic sucrose gradient centrifugation analysis of a crude preparation of membranes of bovine adrenomedullary vesicles. The two peaks of DBH activity corresponded to the peak of catecholamines (CA) and to the peak of the mitochrondrial “marker enzyme” cytochrome c oxidase. The high DBH activity of the latter peak was found to be due to stimulation by mitochondrial contamination. The specific activity of dopamine β-hydroxylase in membranes of highly purified bovine adrenomedullary vesicles was shown to be reduced by approx. 85 per cent upon dilution. This effect of dilution was abolished, and the activity enhanced, by the presence of a mitochondrial fraction. This enhancing effect is probably due partly to nonspecific stimulation of the DBH activity by mitochondrial protein and partly to catalase present in the mitochondrial preparation. A similar effect has been observed with sympathetic nerve vesicles. The implications of this effect for other density gradient studies are discussed.
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