Endoproteases other than furin have a role in hepatic proprotein processing

The enzyme or enzymes responsible for dibasic-directed proprotein processing in the liver have not yet been unequivocally identified, although there are a number of potential candidates. We have compared a Kex2-like proalbumin convertase activity present in rat liver ER/Golgi membranes with recombinant furin, a candidate hepatic convertase. Using a series of mutant recombinant proalbumins as substrates the biochemically identified convertase and furin had very similar specificities with both preferring a substrate with an ArgXaaArgArg processing motif. Kinetic studies with normal and -4R proalbumin suggested however that the proalbumin convertase was not identical to furin. This was confirmed in immunoabsorption studies which demonstrated that furin only accounts for approximately half of the convertase activity. Therefore at least two proprotein convertases with overlapping specificities are involved in hepatic proprotein processing.