Identification of carboxypeptidase N as an enzyme responsible for C-terminal cleavage of stromal cell-derived factor-1α in the circulation

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Abstract
The chemokine stromal-derived factor-1α (SDF-1α) is an essential regulator of hematopoiesis, lymphocyte homing, pre-B-cell growth, and angiogenesis. As SDF-1α is constitutively expressed in many tissues, chemokine function is mostly regulated by proteolytic degradation. Human serum cleaves the 68-amino acid chemokine, SDF-1α, at both termini. The enzyme or enzymes responsible for the removal of the carboxy-terminal lysine from SDF-1α, leading to significant reduction in biologic activity, have not been identified. Using a new biochemical assay for measuring the carboxy-terminal cleavage activity, we purified from serum and plasma a peptidase that specifically removes the carboxy-terminal lysine from SDF-1α and identified it as carboxypeptidase N (CPN, also known as kininase I, arginine carboxypeptidase, and anaphylotoxin inactivator). We demonstrate that SDF-1α in serum and plasma lacks the carboxy terminal lysine, and depletion of CPN from serum and plasma significantly reduces the SDF-1α carboxypeptidase activity. Purified CPN effectively and specifically removes the carboxy-terminal lysine from SDF-1α and significantly reduces the chemokine's biologic activity as a pre-B-cell growth factor and chemoattractant. Thus, in addition to its role as a regulator of the biologic activity of kinins and anaphylatoxins, CPN is an important regulator of the biologic activity of SDF-1α by reducing the chemokine-specific activity. (Blood. 2005;105:4561-4568)