Abstract
The H +/substrate stoichiometries of the tonoplast H +-ATPase and H +-PPase were determined by a kinetic approach. Using red beet ( Beta vulgaris L.) tonoplast vesicles, rates of substrate-dependent H + transport were estimated by (I) a mathematical model describing the time course of ΔpH formation, (II) the rate of H + leakage following H + pump inhibition at a steady state ΔpH, and (III) the initial rate of alkalinization of the external medium. When compared with rates of substrate hydrolysis measured under identical conditions, all three methods yielded an H +/ATP stoichiometry of 2 while the H +/ PP i stoichiometry was determined to be 1 using methods I and II. Experimental limitations did not permit an analysis of the H +/ PP i, stoichiometry by method III. From these results and the estimated level of substrate and product typically found in the cytoplasm of plant cells, it is suggested that the H +-ATPase and H +-PPase as primary H +-pumps are poised toward net substrate hydrolysis under in vivo conditions thereby operating in parallel to generate a proton electrochemical gradient across the tonoplast.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
