Structure and function of human plasma carboxypeptidase N, the anaphylatoxin inactivator

Abstract

Human carboxypeptidase N (CPN) was discovered in the early 1960s as a plasma enzyme that inactivates bradykinin and was identified 8 years later as the major "anaphylatoxin inactivator" of blood. CPN plays in important role in protecting the body from excessive buildup of potentially deleterious peptides that normally act as local autocrine or paracrine hormones. This review summarizes the structure, enzymatic properties and function of this important human enzyme, including insights gained by the recent elucidation of the crystal structure of the CPN catalytic subunit and structural modeling of the non-catalytic regulatory 83 kDa subunit. We also discuss its physiological role in cleaving substrates such as kinins, anaphylatoxins, creatine kinase, plasminogen receptors, hemoglobin and stromal cell-derived factor-1α (SDF-1α). HistoryCarboxypeptidases were initially isolated from pancreatic extracts and so were associated with protein and peptide degradation in the digestive tract [1][2][3][4]. However, work beginning in the early 1960's by Erdös and Sloane [5] revealed the presence of a novel carboxypeptidase in the blood that plays a regulatory role by inactivating bradykinin via removal of its C-terminal Arg residue. (Subsequent research some 2 decades later led to the discovery of a second kinin B 1 receptor activated by the carboxypeptidase metabolite that is its endogenous agonist -see below). Initially it was assumed that the enzyme represents a circulating form of pancreatic carboxypeptidase B, but this was disproved by the characterization of its enzymatic properties [5][6][7][8], which was later borne out by biochemical studies, sequence analysis [9][10][11][12][13][14][15] and ultimately X-ray crystallography [16]. To differentiate it from the pancreatic enzyme, it was named carboxypeptidase N (CPN; EC 3.4.17.3) and was the first member of a family of carboxypeptidases now known as the regulatory or CPN/E subfamily of metallocarboxypeptidases [17][18][19]. It has since been re-discovered many times as other substrates were found and so has acquired aliases such as creatine kinase conversion factor, plasma carboxypeptidase B, arginine carboxypeptidase, lysine carboxypeptidase and protaminase [8,17,20,21]. The most notable was the finding by Bokisch and Müller-Eberhard in 1969 -70 [22,23] that the human plasma "anaphylatoxin inactivator" is identical with CPN. This finding explains our interest in the work of Dr. Tony Hugli, who has made many seminal contributions to the understanding of the structure and function of anaphylatoxins [24][25][26][27]. In Send Correspondence and Proofs To: Randal A. Skidgel, PhD, Dept. of Pharmacology (M/C 868), Univ. of Illinois College of Medicine, 835 S. Wolcott, Chicago, IL 60612, Phone: 312-996-9179, FAX: 312-996-1648, EMAIL: E-mail: rskidgel@uic.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript w...

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