N-Acetyl-Ser-Asp-Lys-Pro, an endogenous tetrapeptide secreted by bone marrow, is a specific substrate for the N-terminal site of ACE. In Vitro: N-Acetyl-Ser-Asp-Lys-Pro is degraded specifically by ACE, and its plasma level rises substantially during ACE inhibitor therapy. Flow cytometry of rat cardiac fibroblasts treated with N-Acetyl-Ser-Asp-Lys-Pro shows significant inhibition of the progression of cells from G0/G1 phase to S phase of the cell cycle. Moreover, phosphorylation and nuclear translocation of Smad2 is decreased in cardiac fibroblasts treated with N-Acetyl-Ser-Asp-Lys-Pro^[1]. N-acetyl-seryl-aspartyl-lysyl-proline appears to exert this function by blocking the action of a stem cell-specific proliferation stimulator and acts selectively on quiescent progenitors^[2]. N-Acetyl-Ser-Asp-Lys-Pro inhibits collagenase expression and activation is associated with increased expression of TIMP-1 and TIMP-2. N-Acetyl-Ser-Asp-Lys-Pro normalizes the IL-1β-mediated increase in MMP-2 and MMP-9 activities and MMP-13 expression^[3]. In Vivo: N-Acetyl-Ser-Asp-Lys-Pro prevents hypertension-induced inflammatory cell infiltration, collagen deposition, nephrin downregulation and albuminuria, which could lead to renoprotection in hypertensive mice^[4].