The processing of human proinsulin and chicken proalbumin by rat hepatic vesicles suggests a convertase specific for X-Y-Arg-Arg or Arg-X-Y-Arg sequences.

Vesicles from rat and chicken livers contain very similar Ca2(+)-dependent proteases that respectively cleave (human) proalbumin at an Arg-Arg site and chicken proalbumin at an Arg-Phe-Ala-Arg site. Similar Ca2(+)-dependent proteases are also present in pancreatic secretory granules and cleave proinsulin at two sites, Arg-Arg and Lys-Arg. The mammalian liver processes a large variety of different proproteins and in order to assess its processing site requirements, we investigated the ability of rat hepatic vesicle extracts to cleave purified chicken proalbumin and human proinsulin. Despite having only a monobasic processing site, chicken proalbumin was cleaved faster than human proalbumin which not only contains a dibasic site, but has an identical propeptide to that of the rat's own proalbumin. Human proinsulin was processed by the rat liver extracts; however, no mature insulin was produced. Cleavage occurred in only one place, presumably the Arg-Arg site at the B-C chain junction. This suggests that the mammalian liver might not contain a Type II Lys-Arg-directed convertase, only a Type I Arg-Arg-specific enzyme. The Type I enzyme that cleaves human proalbumin appears to be the same activity that cleaves chicken proalbumin, suggesting a specificity for either X-Y-Arg-Arg or Arg-X-Y-Arg sequences. This proposal is in keeping with the processing site motif of some 16 different proproteins that are known to be processed in the liver and is entirely consistent with the known in vivo specificity of the enzyme defined by naturally occurring variants of human proproteins.