Subcellular Distribution of Basal and Cation-sensitive ATPases in Roots of Phaseolus vulgaris

A smooth microsomal fraction isolated from homogenates of Pbaseolus vulgaris root tissue has been found to possesss a highly active basal ATPase (measured in the absence of added cations). The microsomal membranes also feature a cation-sensitive ATPase which responds to Mg²⁺, Na⁺ and K⁺, but in a manner that is highly variable with pH. In contrast, membrane fragments prepared by a technique designed to yield purified plasma membrane were capable of little or no hydrolysis of ATP either in the presence or absence of added cations. This suggests that the microsomal activity is a reflection of membrane-bound ATPase which has been derived from cytoplasmic membranes, possibly the tonoplast, rather than plasma membrane.