Detection and biochemical characterization of circulating proteasomes in dog plasma

A growing body of evidence convincingly indicates that proteasomes are not located exclusively within cells but also in different extracellular compartments. In humans, in fact, this large multimeric protease has been identified in many body fluids and secretions such as blood, urine, tears, sweat, saliva, milk, and cerebrospinal and pericardial fluid. Intriguingly, the exact origins of these extracellular proteasomes as well as the specific biological functions they perform are largely unknown. As no data on this important subject is yet available in domestic animals, the present study was undertaken to investigate the presence of extracellular proteasomes in canine blood. As a result, for the first time, circulating proteasomes could be clearly detected in the plasma of a cohort of 20 healthy dogs. Furthermore, all three main proteasomal peptidase activities were measured and characterized using fluorogenic peptides and highly specific inhibitors. Finally, the effect of ATP and PA28 family activators on this circulating proteasome was investigated. Collectively, our data indicate that at least a part of the proteasome present in dog plasma consists of a particle that in vitro displays the enzymatic properties of the 20S proteasome.