Activity of proteoliposomes containing cytochrome oxidase in the submitochondrial orientation

Na(+)-translocating cytochrome bo terminal oxidase from Vitreoscilla: some parameters of its Na+ pumping and orientation in synthetic vesicles

Vitreoscilla cytochrome bo ubiquinol oxidase is similar in some properties to the Escherichia coli enzyme, but unlike the latter, the Vitreoscilla oxidase functions as a primary Na+ pump. When purified Vitreoscilla cytochrome bo is incorporated into liposomes made from Vitreoscilla phospholipids and energized with a quinol substrate, it translocates Na+, not H+, across the vesicle membrane. Since protonophores CCCP (carbonyl cyanide m-chlorophenylhydrazone) and DTHB (3,5-di-tert-butyl-4-hydroxybenzaldehyde) stimulated the Na+ pumping, it is unlikely that it is a secondary effect due to the presence of Na+/H+ antiporter activity in the preparations. The efficiency of the Na+ pumping was 3.93 Na+ pumped per O2 consumed when ascorbate/TMPD was used as the substrate. The cytochrome has a K(m) and Kcat for Na+ of 2.9 mM and 277 s-1, respectively. When ferricytochrome c was entrapped within liposomes prepared from Vitreoscilla phospholipids, it was reduced by Q1H2 (ubiquinol-1) but not by ascorbate/TMPD (N,N,N',N'-tetramethyl-1,4-phenylenediamine). Although Q1H2 was oxidized by cytochrome bo in solution at a rate approximately 14 times that of the latter substrate, the rate of accumulation of Na+ within cytochrome bo vesicles driven by the membrane impermeable ascorbate/TMPD was 1.23 times that of the membrane permeable ubiquinol. These data allowed a calculation that in these synthetic proteoliposomes the cytochrome bo molecules are only 51% directed inward; a value of 61% inward-directed was estimated by measuring the ascorbate/TMPD oxidase activity of the proteoliposomes before and after disrupting them with Triton X-100. A random orientation of the E. coli cytochrome bo oxidase in proteoliposomes has also been reported.

The mechanism of potassium movement across the liposomal membrane

Addition of potassium to sodium-loaded asolectin liposomes induces an internal alkalinization even in the absence of ionophores. Most of the K+ entry is electrogenic, as shown by fluorescent changes in the potential-sensitive probe Oxonol V. The major part of the proton efflux observed must therefore be electrophoretic. However, in the presence of high concentrations of membrane permeable n-butyltriphenylphosphonium, potassium addition induces a residual alkalinization under conditions where no membrane potential can be observed with Oxonol V. This suggests that liposomes also catalyze direct electroneutral K+/H+ exchange, as has been theoretically predicted for cytochrome oxidase proteoliposomes (Wrigglesworth, J.M., Cooper, C.E., Sharpe, M.A. and Nicholls, P. (1990) Biochem. J. 270, 109-118). Free fatty acids present in the soybean phospholipid mixture may be responsible for such activity.