1. Subcellular fractions of human placenta were prepared by nitrogen-bomb homogenization and differential centrifugation. 2. β-Glucuronidase from placental lysosomes was purified 2100-fold on a protein basis. 3. The lysosomal enzyme, at different stages of purification, was characterized by using 4-methylumbelliferyl β-d-glucuronide and phenolphthalein β-d-glucuronide as substrates. 4. Only one isoenzyme of β-glucuronidase was found in placenta; the enzyme in the endoplasmic reticulum appeared to be the same as the lysosomal enzyme. 5. The isoenzyme contained in normal plasma was different from that of the placenta. 6. The elevated β-glucuronidase activity found in plasma obtained during pregnancy was due to increased activity of the normal plasma isoenzyme; no contribution was made by placental isoenzyme. 7. Plasma contained a heat-stable, non-diffusible activator of placental β-glucuronidase. 8. A heat-stable competitive inhibitor of placental and plasma β-glucuronidase was also present in plasma.

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