Rat liver microsomal and lysosomal beta-glucuronidases differ in both carbohydrate and amino acid compositions

To investigate the chemical relationships between rat liver lysosomal and microsomal beta-D-glucuronidases (EC 3.2.1.31), which are essentially identical catalytically and in reactivity with antibody and similar in molecular weight, the two enzymes were isolated by procedures in which modifications of the proteins were avoided. The purified enzymes were found to differ in both sugar and amino acid compositions. The microsomal enzyme contained much more mannose and, in contrast to the lysosomal enzyme, contained sialic acid but no glucose. Moreover, although the amino acid compositions generally agreed closely, the microsomal enzyme contained much more serine and somewhat less arginine than the lysosomal form. These findings of specific differences in composition should have a bearing on the consideration of intracellular glycoprotein synthesis, translocation, and compartmentalization.