Studies on the site of addition of sialic acid and glucosamine to rat α1-acid glycoprotein

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Abstract
Ultrasonic extracts of rough and smooth endoplasmic reticulum fractions 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 assembled mainly on membrane-bound polyribosomes. Incorporation studies with labeled leucine and glucosamine indicates that initial glycosylation of .alpha.1-acid glycoprotein occurs mainly or entirely after release of nascent polypeptide from the ribosomal site.