A unique membrane-bound, calcium-dependent endopeptidase with specificity toward paired basic residues in rat liver...

A unique calcium-dependent endopeptidase specifically cleaving on the carboxyl side of paired basic residues was partially purified from Golgi membrane fractions of rat liver. The enzyme, with optimal pH at around 6.0, hydrolyzes synthetic peptides corresponding to the amino-terminal sequences of proalbumin and proparathyroid hormone at the carboxyl sides of paired basic residues (Arg-Arg and Lys-Arg), but peptides corresponding to the amino-terminal sequences of proalbumin variants, in which Arg-Arg at the site of cleavage is replaced by Arg-Gln or His-Arg, are not affected by the enzyme. From its strict substrate specificity and inhibitory spectrum, this enzyme appears to be a novel endopeptidase distinct from trypsin and cathepsin B and may be physiologically involved in proprotein processing.