title: Demonstration of two distinct calcium pumps in human platelet membrane vesicles.
abstract: Membrane vesicles from human platelets were prepared by various disruption and isolation techniques reported in the literature to yield fractions of predominantly surface or intracellular membrane origin. ATP + Mg2+-dependent Ca2+ accumulation and the formation of acylphosphate intermediates of the calcium pump(s) were followed in parallel experiments, and the consequences of a limited proteolysis of the membranes examined. In all types of preparations active Ca2+ uptake had both oxalate-sensitive and insensitive fractions and calmodulin had no effect on the rate of Ca2+ uptake. Limited proteolysis by trypsin eliminated oxalate-sensitive Ca2+ uptake while it had no effect on the oxalate-insensitive fraction. The Ca2+-induced EP complex had an apparent molecular mass of 100-110 kDa in all of the preparations, the EP showing a broad or even duplicated line in most autoradiographies. Mild trypsin digestion resulted in the formation of 80-, 55-, and 35-kDa phosphorylated fragments. The 80-kDa fragment corresponded to the limit polypeptide found in the proteolyzed erythrocyte membrane Ca2+ pump, its phosphorylation was stimulated by lanthanum, and it appeared in a different time course than the smaller fragments. The molecular mass and the formation pattern of the latter species corresponded to the tryptic fragments in the sarcoplasmic reticulum Ca2+ pump. Based on these results we suggest that platelet membrane preparations contain two types of Ca2+ pump proteins, one similar to the sarcoplasmic reticulum-type and the other to the erythrocyte-type enzyme.
mesh_terms: D000251:Adenosine Triphosphatases / Q000097:blood; D000255:Adenosine Triphosphate / Q000494:pharmacology; D001792:Blood Platelets / Q000378:metabolism* / Q000648:ultrastructure; D002118:Calcium / Q000097:blood* / Q000494:pharmacology; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D006801:Humans; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D007811:Lanthanum / Q000494:pharmacology; D008274:Magnesium / Q000494:pharmacology; D008854:Microscopy, Electron; D010070:Oxalates / Q000494:pharmacology; D019815:Oxalic Acid; D010710:Phosphates / Q000097:blood; D014357:Trypsin / Q000494:pharmacology
chemical_list: D007473:Ion Channels; D010070:Oxalates; D010710:Phosphates; D007811:Lanthanum; D000255:Adenosine Triphosphate; D019815:Oxalic Acid; D014357:Trypsin; D000251:Adenosine Triphosphatases; D008274:Magnesium; D002118:Calcium