Studies on the site of addition of sialic acid and glucosamine to rat alpha 1-acid glycoprotein.

Ultrasonic extracts of rough and smooth endoplasmic reticulum fraction and Golgi fractions from rat liver were examined by immunoelectrophoresis using antiserum to alpha 1-acid glycoprotein. Rough endoplasmic reticulum fractions contained only sialic acid free alpha 1-acid glycoprotein, whereas smooth endoplasmic reticulum and Golgi fractions also contained sialic acid containing alpha 1-acid glycoprotein. Determination of the sialic acid contents of immune precipitates isolated from the extracts suggested that the Golgi complex was the main site of addition of sialic acid to alpha 1-acid glycoprotein. Immunological studies on puromycin extracts of polyribosomes showed that polypeptide chains of alpha 1-acid glycoprotein and albumin were assemble mainly on membrane-bound polyribosomes. Evidence is presented from incorporation studies with labelled leucine and glucosamine that initial glycosylation of alpha 1-acid glycoprotein occurs mainly or entirely after release of nascent polypeptide from the ribosomal site.