242 Biochimica et Biophysica Acta, 470 (1977) 242--250 © Elsevier/North-Holland Biomedical Press BBA 77818 SMALL-ANGLE X-RAY SCATTERING FROM MITOCHONDRIA CARMEN A. MANNELLA * and DONALD F. PARSONS ** Electron Optics Laboratory, Roswell Park Memorial Institute, Buffalo, N.Y. 14263 (U.S.A.) (Received March 9th, 1977) Summary X-ray (CuK,) scattering curves of rat liver mitochondria are characterized by continuously decreasing intensity from 0.5 to 5 mrad and a broad maximum centered near 20 mrad. The condensed-to-orthodox morphological transition of the inner membranes of intact mitochondria causes a dramatic decrease in scat- tering at very small angle and a marked shift of the 20 mrad maximum to smaller angle. A similar small-angle scattering maximum is observed with inner mitochondrial membrane fractions prepared by digitonin treatment and osmo- tic shock/step gradient centrifugation procedures. However, the small-angle X- ray scattering curves of mitochondria after acetone treatment and osmotic lysis/sonication are essentially continuous. These characteristics of mitochon- drial X-ray scattering are discussed in terms of known structural features of the organelle. Introduction The mitochondrion plays a central role in eukaryote energy metabolism by virtue of its inner membrane oxidative phosphorylation process. Thus, despite the structural complexity of this organelle, it has been the focus of many studies relating biomembrane structure to function. Electron microscopy and light scattering, for example, have revealed that close correlations exist between mit- ochondrial morphology and respiratory state [1--4]. Such findings have been interpreted by some in terms of direct involvement of cristal membrane confor- mation in the events of energy transduction [2,5]. Likewise, changes in the fluorescence intensity of the dye 8-anilinonaphthalene-l-sulphonate, when in- * Present Address: St. Louis University School of Medicine, Biochemistry Department, 1402 South Grand Blvd., St. Louis, Mo. 63104, U.S.A. ** Present Address: Division of Laboratories and Research, New York State Department of Health, Empire State Plaza, Albany, N.Y. 12201, U.S.A.