Plasma anticoagulant proteins, such as AT; protein C; protein S; and the fibrinolytic protein, plasminogen, are important for maintenance of hemostatic balance and for protecting against thromboembolism. Although information regarding the clinical use of measurements of AT and fibrinolytic proteins is available in the veterinary literature, there is far less information concerning the vitamin K–dependent anticoagulants, protein C and protein S.1–3
Protein C is a disulfide-linked glycoprotein with a molecular weight of 62 kd, similar to the molecular weight of albumin. Protein C is synthesized in the liver and circulates as a plasma zymogen, with conversion to an active serine protease occurring primarily at the luminal surface of endothelial cells. Activation occurs by interaction of protein C with thrombin bound to its transmembrane cofactor, thrombomodulin; this process is enhanced when protein C is bound to an adjacent membrane receptor (endothelial cell protein C receptor; Figure 1). Activated protein C is released from its receptor and combines with protein S through interactions with phospholipids on platelet and endothelial cell membranes. The APC–protein S complex exerts its anticoagulant effect by degrading 2 coagulation cofactors, factors Va and VIIIa, that are essential for sustained generation of thrombin and the formation of a fibrin clot (Figure 2). The plasma half-life of the zymogen protein C is approximately 6 hours, with rapid (15-minute) inactivation of APC occurring by plasma protease inhibitors.4 In addition to its major anticoagulant action, APC has a diverse repertoire of biologic effects, including promotion of fibrinolysis, modulation of inflammation, and inhibition of apoptosis.4–6
Low protein C activity has been associated with thrombotic disorders in humans and animals.4,5,7 Hereditary protein C deficiency is a risk factor for venous thrombosis in humans, with heterozygotes having a 7- to 8-fold increased risk.8 Homozygous protein C deficiency is rarely diagnosed; however, the trait manifests as severe cutaneous and CNS thrombosis in the syndrome of neonatal purpura fulminans.4,9-11 Protein C deficiency more often develops as an acquired disorder because of increased turnover and consumption in the acute phase of inflammation or in association with septicemia and DIC. Hepatic failure and vitamin K deficiency also may profoundly reduce levels of functional protein C activity because of decreased synthesis and a lack of posttranslational modification (γ-carboxylation), respectively.4,5 Low protein C activity also develops in patients with portal vein thrombosis; however, the pathogenesis of this deficiency is not welldefined and may represent an epiphenomenon rather than a causal factor in thrombus formation.9
Beyond the characterization of thrombotic syndromes, measurement of plasma protein C activity has been used clinically to assess hepatic function in human patients with diverse hepatic disorders.12 Low protein C activity has been described in patients with inflammatory hepatopathies, cirrhosis, portal venous obstruction, and infiltrative and neoplastic diseases and has been used as a prognostic indicator for monitoring hepatic transplant patients after surgery.12–17
In our preliminary studies of protein C activity in dogs, we discovered that many dogs with acquired and congenital hepatobiliary disorders had low protein C activity and that dogs with PSS appeared to develop the lowest protein C activity.a The purpose of the study reported here was to evaluate the diagnostic use of protein C for detection of hepatobiliary disease and PSS in dogs. We suspected that protein C deficiency was a common feature of hepatobiliary disease and that inclusion of protein C analyses with serum biochemical profiles, coagulation profiles (including AT), and determination of TSBA concentrations may help distinguish dogs with PSS. We specifically recruited dogs with congenital PSVA and MVD into the study to challenge this hypothesis.
Activated protein C
Disseminated intravascular coagulation PSS Portosystemic shunting
Total serum bile acids
Portosystemic vascular anomalies MVD Microvascular dysplasia
Activated partial thromboplastin time ALT Alanine transaminase
Mean corpuscular volume
Receiver operating characteristic
Toulza O, Center SA, Brooks MB, et al. Protein C deficiency in dogs with liver disease (abstr), in Proceedings. 22nd Annu Am Coll Vet Intern Med Forum 2004;866–867.
Coulter S+ IV electronic counter, Coulter Electronics, Hialeah, Fla.
Hitachi 911, Boehringer Mannheim, Indianapolis, Ind.
STA Compact, Diagnostica Stago, Parsippany, NJ.
Dade Actin FS, Dade Behring, Durham, NC.
Thromboplastin LI, Helena Diagnostics, Beaumont, Tex.
Fibriquik Bovin Thrombin, bioMerieux, Durham, NC.
Fibrinogen, Diagnostica Stago, Parsippany, NJ.
STAchrom ATIII, American Bioproducts, Parsippany, NJ.
STAchrom Protein C, American Bioproducts, Parsippany, NJ.
Statistix, version 7.0, Analytical Software, Tallahassee, Fla.
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Faust SN, Heyderman RS, Levin M. Coagulation in severe sepsis: a central role for thrombomodulin and activated protein C. Crit Care Med 2001;29 (suppl 7):S62–S68.
Mack CL, Superina RA, Whitington PF. Surgical restoration of portal flow corrects procoagulant and anticoagulant deficiencies associated with extrahepatic portal vein thrombosis. J Pediatr 2003;142:197–199.
Pinto RB, Silveira TR, Bandinelli E, et al. Portal vein thrombosis in children and adolescents: the prevalence of hereditary thrombophilic disorders. J Pediatr Surg 2004;39:1356–1361.
Dubuisson C, Boyer-Neumann C, Wolf M, et al. Protein C, protein S and antithrombin III in children with portal vein obstruction. J Hepatol 1997;27:132–135.
Schermerhorn T, Center SA, Dykes NL, et al. Characterization of hepatoportal microvascular dysplasia in a kindred of Cairn terriers. J Vet Intern Med 1996;10:219–230.