LIGHT-SCATTERING STUDIES ON RABBIT BRAIN MICROSOMES .2. EFFECTS OF ATP AND CHELATION OF MG2+ ON MICROSOMAL CONTRACTION

Addition of ATP to a microsomal suspension in 0.25 M sucrose containing K+ caused a significant decrease in light scattering from the suspension. By comparing such an ATP-induced change in the angular scattering pattern with that of osmotically induced microsomal shrinking, as well as in relation to changes in microsomal water content, the ATP-induced decrease in light scattering was proved to be due to the shrinkage of microsomal vesicles. The effect of ADP was nearly identical to that of ATP, while AMP showed practically no effect. In view of the fact that both active nucleotides are capable of chelating Ca2+ and Mg2+, various chelating agents such as EDTA were examined and were found to cause a decrease in light scattering, the extent of which varied linearly with the logarithm of their association constant for Mg2+. Such a rule was found to apply to ITP and AMP, but not to ATP and ADP. Moreover, the presence of Na+, K+, Li+ or Rb+ in the suspension was required for the occurrence of such a specific effect of ATP and ADP. Thus it was concluded that factors other than their action in chelating Mg2+ contribute to the contraction of microsomal vesicles induced by ATP and ADP. The action of these nucleotides was not affected by ouabain but was depressed by p-chloromercuribenzoate, indicating that some enzymic mechanism might be involved. © 1969.