Energetics of Proline Transport in Corn Mitochondria

The mechanism of proline entry into the matrix region of isolated corn mitochondria (Zea mays L. Mo17 x B73) was investigated by measuring osmotically induced changes of mitochondrial size (changes in A(520)) in combination with oxygen uptake measurements. Using NADH oxidation to generate the electrochemical gradient, we have determined that proline transport is stereospecific and that it can be inhibited by the proline analog l-thiazolidine-4-carboxylic acid.The energetics of proline transport was investigated by measuring the effects of FCCP (p-trifluoromethoxycarbonyl cyanide phenylhydrazone) and valinomycin on mitochondrial swelling and substrate oxidation. Proline transport and resulting oxidation were found to be partially dependent upon the energy of the electrochemical gradient. At low proline concentrations, entry was found to be primarily independent of the gradient (based on insensitivity to FCCP), whereas at higher proline concentrations a gradient-dependent mechanism became involved. Results with valinomycin indicated that proline transport and oxidation are dependent upon the pH potential across the membrane rather than the electrical (membrane) potential.