Ca2+-mediated catabolism of human erythrocyte band 3 protein

The role of calpain and calpastatin in the catabolism of erythrocyte-membrane proteins during anaemia in hamsters (Mesocricetus auretus) infected with Leishmania donovani

The anaemia associated with visceral leishmaniasis is accompanied by altered Ca(2+) homeostasis and degradation of the cytoskeletal and integral proteins of the erythrocytic membrane. In the present study, such changes were followed in hamsters that were anaemic as the result of their experimental infection with Leishmania donovani. At each stage of the infection, the blood concentration of haemoglobin was found to be negatively correlated with the concentration of Ca(2+) (R(2) = 0.91), the percentage of erythrocytes with Heinz bodies (R(2) = 0.98) and thiol depletion (R(2) = 0.96) in the erythrocytes. Calpain (Ca(2+)-activated protease; EC 3.4.22.17) and its natural inhibitor calpastatin are known to regulate the catabolism of membrane structural proteins. Densitometric scanning of SDS-PAGE gels showed that erythrocytic membranes from infected hamsters contained less calpain and calpastatin than those from control animals. The level of calpain autolysis was found to increase as the infection progressed. The addition of purified calpain (from control hamsters) to erythrocyte ghosts caused greater degradation of the membranes of erythrocytes from infected animals than of the corresponding membranes from control animals. Calpastatin from the control hamsters was more effective, at inhibiting calpain-induced membrane proteolysis, than calpastatin from the infected animals. The results indicate that the Ca(2+)-activated protease and its inhibitor are involved in the degradation of erythrocytic membranes observed during visceral leishmaniasis.

Sulfate self-exchange and amino acid transport in calcium-loaded human erythrocytes

To analyze the effects of Ca2(+)-mediated membrane protein changes on the membrane function, we have studied the SO4(2-) self-exchange and amino acid transport in human erythrocytes after loading them with Ca2+ with the help of ionophore A23187. The SO4(2-) self-exchange is inhibited by 20-30% by loading the erythrocytes with 25 microM to 0.5 mM Ca2+. The extent of this inhibition is almost doubled (50-60%) by increasing the Ca2+ loading concentration to 1.5 mM. This additional effect of 1.5 mM Ca2+ is not correlated with the Ca2(+)-induced ATP depletion or membrane protein degradation, but is caused by the transglutaminase-catalyzed membrane protein crosslinking. Like the SO4(2-) self-exchange, L-alanine and L-cysteine uptakes are also inhibited in Ca2(+)-loaded cells. However, no effect is observed on the L-lysine uptake under identical conditions. These results have been interpreted to suggest that the Ca2(+)-mediated effects on the SO4(2-) self-exchange and amino acid transport are caused perhaps by the Ca2(+)-induced structural rearrangement of the band 3 protein.

Calpains (intracellular calcium-activated cysteine proteinases): structure-activity relationships and involvement in normal and abnormal cellular metabolism

1. Calpains (calcium-activated cysteine proteinases) have evolved by gene fusion events involving calmodulin-like genes, cysteine proteinase genes and other sequences of unknown origin. 2. The enzymes are composed of two non-identical subunits, each of which contains functional calcium-binding sequences. 3. Calpains are inhibited by the endogenous protein inhibitor, calpastatin and some calmodulin antagonists are also inhibitors of calpain. A number of synthetic proteinase inhibitors also inhibit calpains. 4. Calpains can be activated by phospholipids, an endogenous protein activator and some amino acid derivatives. 5. Various protein substrates for calpains have been recognized in vitro, but the identity of in situ substrates remains unclear. 6. Proposals have been made for calpain function, including involvement in signal transduction, platelet activation, cell fusion, mitosis and cytoskeleton and contractile protein turnover. 7. Calpain and calpastatin expression is altered in a number of abnormal states including muscular dystrophy, muscle denervation and tenotomy, hypertension and platelet abnormalities.