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Properties of a plasmalemma atpase of the maize scutellum


, : Properties of a plasmalemma atpase of the maize scutellum. Phytochemistry (Oxford) 18(4): 555-560

Properties of a plasmalemma phosphatase of the maize [Zea mays] scutellum, tentatively identified as an ATPase were investigated. Fresh and frozen-thawed scutellum slices that had been treated with 10 mM HCl to destroy acid phosphatases were used as a source of enzyme. With the exceptions of the Na+, K+ and dinitrophenol experiments, the 2 kinds of slices gave similar results. ATP and CTP were the best substrates for the enzyme followed by TTP, UTP, CDP, ADP and GTP, UDP, nucleoside monophosphates, sugar phosphates, PPi and p-nitrophenyl phosphate were relatively ineffective as substrates. The Km for ATP and ADP were 0.65 and 5 mM, respectively, but the 2 substrates gave the same Vmax (49.8 .mu.mol Pi/h per g slices). Previously it was shown that the products of ATP hydrolysis are ADP, AMP and Pi. Using these previous results and from the time courses of ATP disappearance from the bathing solution and the appearance of Pi and ADP, it was concluded that ATP and ADP were hydrolyzed by the same enzyme. The ATPase was not inhibited by oligomycin. N-N'-Dicyclohexylcarbodiimide (DCCD) was a poor inhibitor and a water soluble analog of DCCD, 1-ethyl-3-(3 dimethylaminopropyl)-carbodiimide, gave only 33% inhibition. The relative effectiveness of divalent cations for stimulating ATPase activity was Mn2+ > Mg2+ .gtoreq. Ca2+ > Co2+. Na+ and K+ gave a small additional stimulation in the presence of Mg2+. Na+ and K+ gave a much greater stimulation when no divalent cation was added; this occurred only when fresh slices were used. Dinitrophenol also increased ATPase activity only when fresh slices were used. Both the uptake of Na+ and K+ and the action of dinitrophenol would probably lower the electrochemical gradient of protons across the plasmalemma. The different results obtained with fresh slices indicate that the ATPase in these slices was under the constraint of proton gradient.

US$19.90

DOI: 10.1016/s0031-9422(00)84259-4


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