The calpain system in human placenta

The calpain system is involved in a number of human pathologies ranging from the muscular dystrophies to Alzheimer's disease. It is important, therefore, to be able to obtain and to characterize both mu-calpain and m-calpain from human tissue. Although human mu-calpain can be conveniently obtained from either erythrocytes or platelets, no readily available source of human m-calpain has been described. Human placenta extracts contain both mu-calpain and m-calpain in nearly equal proportions and in significant quantities (3-4 mg mu-calpain and 4-5 mg m-calpain/1000g placenta tissue). Placenta also contains calpastatin that elutes off ion-exchange columns over a wide range of KCl concentrations completely masking the p-calpain activity eluting off these columns and even partly overlapping m-calpain elution. Placenta p-calpain requires 50-70 muM Ca2+ and placenta m-calpain requires 450-460 muM Ca2+ for half-maximal proteolytic, activity. Western analysis of washed placenta tissue shows that placenta contains both mu- and m-calpain, although some of the mu-calpain in whole placenta extracts likely originates from the erythrocytes that are abundant in the highly vascularized placenta. Placenta calpastatin could not be purified with conventional methods. The most prominent form of calpastatin in Western analyses of placenta obtained as soon as possible after birth was similar to48-51 kDa; partly purified preparations of placenta calpastatin also contained 48-51 and 70 kDa polypeptides. Human placenta extracts likely contain two different calpastatin isoforms, a 48-51 kDa placenta calpastatin and a 70 kDa erythrocyte calpastatin. (C) 2002 Elsevier Science Inc.