Tissue fractionation studies. 8. Cellular localization of bound enzymes

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Abstract
Whole-cell suspensions were prepared from the perfused livers of rats injected with iron particles, and further fractionated into reticulo-endothelial cells containing iron, which were separated by means of a magnet, and parenchymatous cells, isolated by centrifuging. In 2 experiments, purified parenchymatous cells were further fractionated into 1 nuclear and 4 cytoplasmic fractions, according to the scheme of Duve et al. (1955). All fractions were analyzed for N, cyto-chrome oxidase, acid phosphatase ribonuclease, deoxyribonuclease, cathepsin, [beta]-glucuronidase, uricase and glycose 6-phosphatase. All investigated enzymes were present in significant amounts in the parenchymatous cells and showed the same distribution amongst the fractions isolated from these cells as was observed previously on unperfused whole liver. The reticulo-endothelial cell fraction also contained all the activities, but in this the possibility that some of the enzymes were derived from contaminating components of parenchymatous origin could not be ruled out. Least open to this objection were the results obtained on ribonuclease, deoxyribonuclease and cathepsin, the specific activity of which was increased in the reticulo-endothelial cell fraction. Applying these results to the problem of the cellular origin of lysosomes, it was concluded that these particles belong essentially to the parenchymatous cells, but that some of their characteristic enzymes may also be present in the reticulo-endothelial cells, a possibility which could account, at least partly, for the apparent heterogeneity of lysosomes commented upon in an earlier paper.