Intracellular and transcellular transport of secretory component and albumin in rat hepatocytes.

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Abstract
The intra- and transcellular transports of hepatic secretory and membrane proteins were studied in rats in vivo using [3H]fucose and [35S]cysteine as metabolic precursors. Incorporated radioactivity in plasma, bile and liver subcellular fractions was measured and the labeled proteins of the Golgi complex, bile and plasma were separated by SDS PAGE [sodium dodecyl sulfate polyacrylanide gel electrophoresis] and identified by fluorography. 3H-radioactivity in Golgi fractions peaked at 10 min postijection (p.i.) and then declined concomitantly with the appearance of labeled glycoproteins in plasma. Maximal secretion of secretory fucoproteins from Golgi membrane subfractions occurred past 30 min p.i., indicating that membrane proteins leave the Golgi complex at least 30 min later than the bulk of content proteins. A major 80,000-dalton form of secretory component (SC) was identified in the bile by coprecipitation with (IgA)2 by an anti-IgA antibody. An antibody (raised in rabbit) against the biliary 80,000-dalton peptide recognized 2 larger forms (116,000 and 94,000 dalton), presumably precursors, in Golgi membranes. A comparative study of kinetics of transport of 35S-SC and 35S-albumin showed that albumin peaked in bile at .apprx. 45 min p.i., whereas the SC peak occurred at 80 min p.i., suggesting that the transit time differs for plasma and membrane proteins that are delivered to the bile canaliculus.