Abstract
Vacuoles isolated from mesophyll protoplasts of the Crassulacean acid metabolism plant Bryophyllum daigremontianum take up [14C]l-malic acid from the medium. The initial rates of uptake show saturation kinetics (apparent Km about 1 millimolar). In competition experiments, the uptake rates of l-malic acid are greatly reduced in the presence of a surplus of d-malic and tartronic acid. A number of other acids are much less effective and aspartic and isocitric acid have virtually no effect. The uptake rate of malic acid is unchanged in the presence of ATP and the uncouplers dinitrophenol and carbonyl cyanide m-chlorophenyl hydrazone. When the vacuoles are isolated in the absence of bovine serum albumin or when 1 millimolar HgCl2 is added, they are inactive in transport but retain their malic acid pool. The existence of a specific permease which catalyzes an exchange diffusion of malic acid across the vacuolar membrane is postulated.
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