Transient up-regulation of a prolyl endopeptidase activity in the microsomal fraction of rat liver during postnatal development

To identify proteases which are involved in cell proliferation and differentiation of liver cells, we assayed various protease activities in rat liver during its postnatal development. We found that a protease activity specific to proline residues in the microsomal fraction of rat liver was transiently increased around postnatal day 8 and decreased thereafter, indicating that the enzyme activity is highly correlated to the proliferation and differentiation of liver cells. This protease was purified to an apparent homogeneity from the microsomal fraction of the postnatal-day-8 rat liver. The purified enzyme gave a single band with an apparent molecular mass of 65 000 on SDS/PAGE. It cleaved several peptide 4-methylcoumaryl-7-amide substrates and bioactive peptides at the C-terminus of proline residues. The enzyme did not hydrolyze any protein substrate examined suggesting that it is a peptidase rather than a proteinase. Although the best substrate among those tested was succinyl-Gly-Pro-Leu-Gly-Pro-4-methylcoumaryl-7-amide (-NH-Mec), the purified enzyme did not hydrolyze succinyl-Gly-Pro-NH-Mec, indicating that the enzyme has different substrate specificity from any hitherto known prolyl endopeptidases. These results suggest that the purified enzyme is a unique prolyl endopeptidase which may be involved in proliferation and differentiation of liver cells.