Alterations in density and size distribution of intramembrane particles in the inner membrane of mitochondria from chloramphenicol-fed mice.

The diameters and number per unit area (density) of intramembrane particles in mitochondrial inner membranes isolated from livers of chloramphenicol-fed and control mice were assessed by freeze-fracture electron microscopy. In addition, the levels of cytochrome hemes aa3, b, and c + c1 of the mitochondria were measured spectrophotometrically. The results show that CAP caused (1) a portion of the mitochondria to develop much larger than normal matrix volumes and inner membrane surface areas, (2) a decrease in the average diameter of intramembrane particles on both the convex and concave fracture faces of all membranes regardless of the membrane surface area, (3) a decrease in intramembrane particle density on only those membranes with a greater than normal surface area, and (4) a 75% decrease in heme aa3, a 25% decrease in heme b, and no change in heme c + c1 levels. It is concluded that intramembrane particles represent membrane integral proteins and that the components of integral proteins which originate from mitochondrial translation are located in the hydrophobic region of the bilayer and occupy volumes shared equally by each bilayer leaflet. In addition the mechanism by which CAP induces a portion of the mitochondria to develop larger than normal sizes most likely includes a net lipid insertion into the bilayer which exceeds the net protein insertion during the CAP-feeding interval.