Structural changes in microsomal suspensions: V. Interactions with nucleotides

Abstract

Brain microsomes incubated with 3 m m ATP or cytidine triphosphate (CTP) exhibited diminished changes in light scattering during incubation in a hypotonic medium; this difference was abolished on subsequent addition of the sulfhydryl reagent, p-chloromercuribenzoate (pCMB). Incubation with 3 m M ADP had little effect on the spontaneous changes, but significantly increased changes in light scattering induced by pCMB, thereby demonstrating a second type of structural change. Preincubation of the microsomes with pCMB prevented all these changes, and greatly reduced the binding of ADP to the membranes. These two types of changes in light scattering could be correlated with changes in permeability to sucrose: ATP, but not ADP, decreased sucrose permeability; on the other hand, preincubation with ADP, but not ATP, increased sucrose permeability during subsequent incubation with pCMB. ADP, and to a lesser extent ATP, reduced the content of reactive sulfhydryl groups in the microsomal preparation, suggesting that the structural changes involved a nucleotide-membrane-sulfhydryl system in which ADP reduced the content of reactive sulfhydryl groups while sensitizing the system to sulfhydryl reagents. Possible relevance to the transport ATPase is suggested in which the substrate of the enzyme, ATP, induces one structural change, whereas ADP, a product, induces a second structural state in the enzyme-membrane complex.