An accessory protein identical to mouse egasyn is complexed with rat microsomal beta-glucuronidase and is identical to rat esterase-3.

We report biochemical, immunological, and genetic studies which demonstrate that an accessory protein with the essential features of mouse egasyn is complexed with and stabilizes a portion of beta-glucuronidase in microsomes of rat liver. The accessory protein exists as a complex with beta-glucuronidase since it coprecipitates with beta-glucuronidase after treatment of extracts with a specific beta-glucuronidase antibody. The two proteins are associated by noncovalent bonds since they are easily dissociated at elevated temperatures. Only 20-25% of total liver accessory protein is complexed with microsomal beta-glucuronidase. The remainder exists as a free form. The molecular weight of the accessory protein is 61 to 63 kDa depending upon the rat strain of origin. This protein, like mouse egasyn, has esterase catalytic activity and is concentrated in microsomes. The accessory protein is genetically polymorphic with at least four alleles. Combined biochemical and genetic evidence indicates it is identical with esterase-3 of the rat. Also, both mouse egasyn and rat esterase-3 react with antisera to egasyn and to rat esterase-3, indicating they are homologous proteins. Several inbred rat strains lack microsomal beta-glucuronidase. The same strains lack the accessory protein, suggesting that stabilization of beta-glucuronidase in rat microsomes requires egasyn.