The localization of an intracellular membrane-bound proteinase from rat liver

To localize the membrane-bound, histone-degrading proteinase, which was previously isolated from the mitochondrial fraction, nuclei, mitochondria, lysosomes, peroxisomes, smooth and rough endoplasmic reticulum, ribosomes and plasma membranes were prepared from a rat liver homogenate according to established methods and characterized by marker enzyme activities. The isolated subcellular fractions were treated with digitonin, and subjected to a discontinuous sucrose gradient centrifugation. The material, which sedimented through 1.74 M surcrose was analyzed in respect to the various marker enzymes and for proteolytic activity. Proteinase activity was found in the material obtained after digitonin treatment and step gradient centrifugation of mitochondria. This finding shows the occurrence of a proteinase in mitochondria; After fractionation of mitochondria into outer and inner membrane, intermembrane fraction and matrix, the proteinase could be localized exclusively in the inner mitochondrial membrane. A possible physiological function of the enzyme during the biosynthesis of inner membrane constituents is discussed.