The Synthesis of Rat Liver Lysosomes1

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Abstract
In order to trace the route of intracellular transport of lysosomal enzymes, β-glucuronidase [EC 3.2.1.31] in the endoplasmic reticulum and the lysosomes was purified from rat liver and the sequence of appearance of the newly synthesized enzyme was followed with respect to these organlles after labeling the enzyme with L-[3H]leucine. β-Glucuronidase was purified from the microsomal fraction and the mitochondrial-lysosomal fraction by organic solvent fractionation, gel filtration, isoelectric focusing, and acrylamide gel-sucrose gradient electrophoresis. The purified enzyme appeared homogeneous on electrophoresis in polyacrylamide gel. After the injection of the labeled amino acid, the microsomal β-glucuronidase rose to a peak of specific radioactivity at 9 h. The activity then decreased up to 18 h and the decreased activity was maintained up to 24 h. Between 9 and 18 h after administration, when the specific radioactivity of the microsomal enzyme was decreasing, an increase in the activity of the lysosomal enzyme was observed and the maximum value was maintained up to 24 h. The time-course of the appearance of labeled β-glucuronidase in these organelles after administration of the labeled amino acid suggests that the newly synthesized enzyme is transported from the endoplastic reticulum to the lysosomes, probably through the Golgi complex. As the appearance of the peak of radioactivity of β-glucuronidase in the microsomes was delayed, an alternate pathway of enzyme transport is also discussed.

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