The activity of dopamine-stimulated adenylate cyclase from rat brain striatum is modulated by temperature and the bilayer-fluidizing agent, benzyl alcohol

Abstract

Benzyl alcohol achieved a marked activation of the adenylate cyclase activity in a partially purified membrane preparation from rat brain striata, although inhibition resulted at high concentrations. The degree of activation observed depended on the ligand used to stimulate the enzyme, with that observed in the presence of guanosine 5''-[.beta.,.gamma.-imido]triphosphate (p[NH]ppG) (5.8-fold) > dopamine + p[NH]ppG (5-fold) > GTP (3-fold) > dopamine + high GTP (2.25-fold) > dopamine (+ low GTP) = basal (+ low GTP) (1.7-fold). The differences in the concentration-dependence of both the activation and inhibition of dopamine-stimulated and basal activities of the enzyme meant that increasing benzyl alcohol concentrations caused a net elevation in the fold-stimulation of the basal activity by dopamine. Arrhenius plots of p[NH]ppG-, GTP-, fluoride-, dopamine-plus-high GTP- and dopamine-plus-p[NH]ppG-stimulated activities all exhibited a single break occurring at around 22.degree. C. This break point was decreased to around 13.degree. C when 50 mM-benzyl alcohol was added to the assays. In the presence of dopamine (+ low GTP), Arrhenius plots exhibited 2 distinct breaks, one .apprx. 21.degree. C and the other .apprx. 11.degree. C. When benzyl alcohol (50 mM) was added to these assays of dopamine (+ low GTP)-stimulated activity, a single break at .apprx. 14.degree. C was observed. For the basal activity the Arrhenius plot exhibited a single break at around 15.degree. C both in the presence and in the absence of 50 mM-benzyl alcohol. It is suggested that the enzyme is activated by productive collisions between independent mobile entities and that the activity of the enzyme may be regulated by changes in membrane fluidity. The breaks in the Arrhenius plots of all of the ligand-stimulated activities, but not the basal activity, are attributed to lipid-phase separations occurring in either the inner or the outer halves of the bilayer.