Electron microscope examination of subcellular fractions. 3. Quantitative analysis of the microsomal fraction...

Rat liver microsomes and microsomal subfractions isolated by density equilibration were submitted to a quantitative morphological and biochemical analysis. The total area of the endoplasmic reticulum was estimated at 7.3 m(2) per g of liver. The microsome fraction contained 2.8 mg of phospholipids and 6.7 mg of proteins per m(2) of membrane area. After correction for ribosomal and intracisternal proteins, the latter value was lowered to 4.7 mg of membrane protein per m(2). More than half of the microsomal vesicles carried ribosomes. After density equilibration of the microsomes, the distribution pattern of ribosomes followed closely that of RNA. The ribosome load of the microsomal vesicles increased steadily along the density gradient, indicating the existence of a continuous spectrum of microsomal entities ranging from entirely ribosome-free vesicles to vesicles heavily coated with ribosomes.