K+ Stimulation of ATPase Activity Associated with the Chloroplast Inner Envelope

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Abstract
Studies were conducted to characterize ATPase activity associated with purified chloroplast inner envelope preparations from spinach (Spinacea oleracea L.) plants. Comparison of free Mg(2+) and Mg.ATP complex effects on ATPase activity revealed that any Mg(2+) stimulation of activity was likely a function of the use of the Mg.ATP complex as a substrate by the enzyme; free Mg(2+) may be inhibitory. In contrast, a marked (one- to twofold) stimulation of ATPase activity was noted in the presence of K(+). This stimulation had a pH optimum of approximately pH 8.0, the same pH optimum found for enzyme activity in the absence of K(+). K(+) stimulation of enzyme activity did not follow simple Michaelis-Menton kinetics. Rather, K(+) effects were consistent with a negative cooperativity-type binding of the cation to the enzyme, with the K(m) increasing at increasing substrate. Of the total ATPase activity associated with the chloroplast inner envelope, the K(+)-stimulated component was most sensitive to the inhibitors oligomycin and vanadate. It was concluded that K(+) effects on this chloroplast envelope ATPase were similar to this cation's effects on other transport ATPases (such as the plasmalemma H(+)-ATPase). Such ATPases are thought to be indirectly involved in active K(+) uptake, which can be facilitated by ATPase-dependent generation of an electrical driving force. Thus, K(+) effects on the chloroplast enzyme in vitro were found to be consistent with the hypothesized role of this envelope ATPase in facilitating active cation transport in vivo.