Immunochemical study and acinar localization of AM1, a murine submandibular glycoprotein with esterolytic activity

Glycoprotein AM₁, a glycoprotein from the submandibular glands of the mouse was isolated from the 100 000 × g tissue extract by polyacrylamide gel electrophoresis. An antiserum to purified glycoprotein AM₁ was prepared, and its specificity was tested by immunodiffusion and immunoelectrophoresis. Glycoprotein AM₁ could be detected in large quantity only in the submandibular glands of the mouse and in very small amounts in the parotid and sublingual glands and in serum. No glycoprotein AM₁ was found in the murine brain, heart, lung, liver, spleen, kidney, pacreas, spinal cord and testis. In addition, glycoprotein AM₁ was not detectable in the submandibular glands of the rat and rabbit, and in whole human saliva. No cross-reactivity was found with murine submandibular proteinase A and porcine pacreatic kallikrein. The cellular localization of glycoprotein AM₁ was determined by the indirect immunofluorescence technique. In the submandibular glands bright fluorescence was only present in the acinar cells, throughout the whole gland. In the sublingual glands faint fluorescence was detectable as a diffuse network around the acini and possibly in the serous acinar demilune cells. On a subcellular level, glycoprotein AM₁ could be demonstrated in the extract of the SM_C secretory granular fraction, which originates largely from the acinar cells. On the other hand, glycoprotein AM₁ was hardly detectable in the SM_B secretory granular fraction, which originates predominantly from the granular convoluted tubular cells. Consomitantly, glycoprotein AM₁ was secreted in vivo and could be detected in whole saliva, particularly after stimulation with isoproterenol and carbamylcholine, and also with phenylephrine, but to a much lesser extent.