531 Biochimica et Biophysica Acta, 555 (1979) 531--546 © Elsevier/North-Holland Biomedical Press BBA 78464 THE TRANSPORT OF L-CYSTEINESULFINATE IN RAT LIVER MITOCHONDRIA FERDINANDO PALMIERI, ITALO STIPANI and VITO IACOBAZZI Department of Biochemistry and C.N.R. Unit for the Study of Mitochondria and Bioenergetics, University of Bari (Italy) (Received January 2nd, 1979) Key words: Cysteinesulfinate transport; Aspartate-glutamate carrier; (Rat liver mitochondria) Summary 1. The mechanism of L-cysteinesulfinate permeation into rat liver mito- chondria has been investigated. 2. Mitochondria do not swell in ammonium or potassium salts of L-cysteine- sulfinate in all the conditions tested, including the presence of valinomycin and/or carbonylcyanide p-trifluoromethoxyphenylhydrazone. 3. The activation of malate oxidation by L-cysteinesulfinate is abolished by aminooxyacetate, an inhibitor of the intramitochondrial aspartate aminotrans- ferase, it is not inhibited by high concentrations of carbonylcyanide p-trifluoro- methoxyphenylhydrazone (in contrast to the oxidation of malate plus glutamate) and it is decreased on lowering the pH of the medium. 4. All the aspartate formed during the oxidation of malate plus L-cysteine- sulfinate is exported into the extramitochondrial space. 5. Homocysteinesulfinate, cysteate and homocysteate, which are all good substrates of the mitochondrial aspartate aminotransferase, are unable to activate the oxidation of malate. Homocysteinesulfinate and homocysteate have no inhibitory effect on the L-cysteinesulfinate-induced respiration, whereas cysteate inhibits it competitively with respect to L-cysteinesulfinate. 6. In contrast to D-aspartate, D-cysteinesulfinate and D-glutamate, L-aspar- tate inhibits the oxidation of malate plus L-cysteinesulfinate in a competitive way with respect to L-cysteinesulfinate. Vice versa, L-cysteinesulfinate inhibits the influx of L-aspartate. 7. Externally added L-cysteinesulfinate elicits efflux of intramitochondrial Abbreviations: MalNEt, N-ethylmaleimide; Mops, 3-(N-morpholino)propanesulfonic acicl; FCCP, car- bonylcyanide p-trifluoromethoxyphenylhydrazone; TMPD, N,N,N',N'-tetramethyl-1,4-phenylenediamine.