Subcellular fractionation of WI-38 fibroblasts. Comparison between young and old cells

WI-38 fibroblasts cultivated in vitro were homogenized and their subcellular organelles analysed by the techniques of differential centrifugation and isopycnic equilibration in density gradient. In these experiments, the assayed enzymes were known to be specifically associated with subcellular components in other cells types. In most cases, their behaviour and properties corresponded with observations made in earlier studies and we could consider them as being representative of the specific subcellular organelles. Some significant differences were observed between young and old fibroblasts. The specific activity of alkaline phosphodiesterase was lower in the old cells whereas for the other enzymes it was identical or higher, especially for the 5'-nucleotidase; also the particulate fractions obtained by differential centrifugation contained more material. After equilibration in density gradient, the average density of the 5'-nucleotidase, alkaline phosphodiesterase and N-acetyl-beta-D-glucosaminidase was less in the old than in the young cells, whereas that of the galactosyltransferase of Golgi apparatus was greater. For mitochondria, endoplasmic reticulum and peroxisomes, the differences observed were small.