A Spin‐Label Study of the Chromaffin Granule Membrane

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Abstract
The structure of the chromaffin granule membrane has been probed using a number of different spin labels. Both the effect of temperature and high levels of calcium have been studied. 1 The results from three positional isomers of the stearic acid spin label demonstrate that a substantial part of the membrane lipid (that is sensed by the probe) is in a bilayer structure which undergoes a structural transition at 32–36 °C, characterized by an increase in the population of gauche isomers in the lipid chains. A possible mechanism for this transition would be the preferential segregation of cholesterol. 2 The covalently bound iodoacetamide spin label reveals a transition within the protein component of the membrane or its immediate lipid environment at 32°C. This transition corresponds to an increased degree of motional freedom of the spin label above the transition temperature. 3 The lipid‐soluble spin label 2,2,6,6‐tetramethyl‐piperidine‐1‐oxyl exhibits a break at 34°C in the temperature‐dependence of its partitioning into the membrane. This could correspond to the onset of a lateral separation in the membrane lipid, again possibly involving a re‐distribution of cholesterol. 4 Calcium abolishes, diminishes or shifts the transition observed by the spin label and decreases the amplitude of motion of the stearic acid spin labels, again possibly involving a restribution of cholesterol and also lysolecithin. The temperatures of the structural transition agree well with the changes in the enzymic activity of the membrane ATPase and NADH oxidase functions and also with the results from fluorescent probes [Bashford et al., Eur. J. Biochem. 67, 105–114 (1976)]. It is possible that triggering of the transition either by calcium or some other stimulus may play a role in catecholamine release and membrane fusion.

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