Immobilization and assay of low-molecular-weight phosphomannosyl receptor in multiwell plates

Abstract

A novel sensitive binding assay for quantitation of a low-molecular-weight phosphomannosyl receptor (41-46 K) was devised. The receptor was immobilized by immunochemical means in the wells of polystyrene multiwell plates. The lysosomal enzyme ligand, testicular [beta]-galactosidase, was added and receptor-bound [beta]-galactosidase was measured by conventional colorimetric analysis using p-nitrophenyl [beta]-galactoside as substrate. Inhibitors of the binding of [beta]-galactosidase to the receptor were removed prior to addition of [beta]-galactosidase and did not interfere with the assay. Low-molecular-weight phosphomannosyl receptor was readily quantitated in the range of 4 to 100 ng of receptor protein. Binding of [beta]-galactosidase to the receptor was specifically inhibited by 5 m mannose 6-phosphate. The receptor exhibited optimum binding of [beta]-galactosidase at pH 5.7 and was saturated with [beta]-galactosidase at 320 munits/ml in the presence of 20 m MnCl2. The requirement for MnCl2 was greatly diminished at higher concentrations of [beta]-galactosidase. Application of the assay procedure to the quantitation of the low-molecular-weight phosphomannosyl receptor in mammalian tissues is discussed.