Immunocytochemical localization of albumin in the secretory apparatus of rat liver parenchymal cells.

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Abstract
The localization of albumin was investigated in rat liver, fixed by perfusion, with peroxidase-labeled monospecific antibodies against rat serum albumin purified by affinity chromatography. By light microscopy, albumin is present uniformly in all parenchymal cells with no difference in the intensity of reaction in the different parts of hepatic lobules. By electron microscopy, albumin is localized in the entire secretory apparatus including the rough and smooth endoplasmic reticulum, Golgi complex, and secretory vacuoles. In the rough endoplasmic reticulum, focal negative segments are interposed between positive regions. In the Golgi region, albumin is found both in stacked cisternae and at the trans aspect in the portion called GERL (Golgi-endoplasmic reticulum--lysosome). Whereas albumin and lipoprotein particles are separated in terminal dilatations of the endoplasmic reticulum and in the cisternae on the cis face of the Golgi apparatus, they are usually intermixed in vacuoles of the trans face. Similarly, most secretion vacuoles below the sinusoidal lining contain albumin and lipoprotein particles together, although a few are also found with only one secretory product. These observations suggest that, after synthesis in the rough endoplasmic reticulum, albumin is segregated into smooth transitional elements and transported to the Golgi region where it is packaged together with other secretory products such as lipoproteins. These secretion vacuoles move up the sinusoidal surface, where they are discharged. The possible involvement of GERL in the proteolytic cleavage of proalbumin to albumin is considered.