Abstract
Amyloplast envelope membranes isolated from cultured, white-wild cells of sycamore (Acer pseudoplatanus L.) have been found to contain a Mg(2+)-ATPase, ranging in specific activity from 5 to 30 nanomoles per minute per milligram protein. This ATPase hydrolyzes a broad range of nucleoside triphosphates, whereas it hydrolyzes nucleoside mono- and diphosphates poorly, if at all. The ATPase activity was stimulated by several divalent cations, including Mg(2+), Mn(2+) and Ca(2+), whereas it was not affected by Sr(2+), K(+), or Na(+). The K(m) for total ATP was 0.6 millimolar, and the activity showed a broad pH optimum between 7.5 and 8.0. The ATPase was insensitive to N,N'-dicyclohexylcarbodiimide and oligomycin, but it was inhibited by vanadate. All these characteristics are basically similar to those reported previously for the Mg(2+)-ATPase of the chloroplast inner-envelope membrane. Likewise, the amyloplast envelope enzyme was shown to be located specifically on the inner envelope membrane. The amyloplast envelope membranes were chemically modified with a series of unique affinity labeling reagents, the adenosine polyphosphopyridoxals (M Tagaya, T Fukui 1986 Biochemistry 25: 2958-2964). About 90% of the ATPase activity was lost when the envelope membranes were preincubated with 0.1 millimolar adenosine triphosphopyridoxal. Notably, the enzyme was protected completely from inactivation in the presence of its substrate, ATP. In contrast, both adenosine diphosphopyridoxal and pyridoxal phosphate caused much less of an inhibitory effect. This greater relative reactivity of the triphosphopyridoxal analog is similar to that reported previously with Escherichia coli F(1) ATPase (T Noumi et al. 1987 J Biol Chem 262: 7686-7692).