ORGANELLAR CLUSTERS FORMED BY MITOCHONDRIAL-ROUGH ENDOPLASMIC-RETICULUM ASSOCIATIONS - AN ORDERED ARRANGEMENT OF MITOCHONDRIA IN HEPATOCYTES

Our objective was to determine if mitochondrial-rough endoplasmic reti culum (mt-RER) associations provide for an ordered arrangement of mito chondria in the cell. If such an ordered arrangement exists, it might be manifested by grouping of mitochondria according to size and bioche mical properties. Liver homogenate was subjected to rate zonal centrif ugation for fractionating mitochondrial clusters, These clusters were then examined for morphological and biochemical characteristics. Scann ing electron microscopy (SEM) showed that (1) mitochondria were held t ogether in clusters by rough endoplasmic reticulum, (2) clusters consi sted of mitochondria of comparable size, and (3) a 45-fold difference in average mitochondrial volume existed between the organelles of the fastest and slowest sedimenting clusters. Transmission electron micros copy (TEM) affirmed that all of the organellar clusters examined were mitochondria associated with rough endoplasmic reticulum. Cytochrome o xidase and mitochondrial DNA were found to be proportional to mitochon drial volume, indicating that these components were synthesized in pro portion to increases in volume. Conversely, succinic dehydrogenase and ornithine carbamoyl transferase were increased disproportionately (2. 9-fold and six-fold, respectively) with increase in mitochondrial volu me, It is evident from this biochemical heterogeneity that clusters co mposed of larger mitochondria differ functionally from clusters of sma ller mitochondria, The size ordered arrangement suggests that this org anization is in some way related to the biogenesis of hepatocyte mitoc hondria, It is also conjectured that the biochemical heterogeneity is a consequence of addition of selected proteins (e.g., succinic dehydro genase and carbamoyl transferase) to mitochondria in a developmental p rocess as they mature into larger organelles.