title: Active calcium transport by porcine thyroid microsomes.
abstract: Microsomal fractions prepared from porcine thyroid glands by differential centrifugations and sucrose density gradient centrifugation showed an ATP-dependent Ca2+ uptake. Electron microscopy and the study of marker enzyme activities suggested that the fractions consisted mainly of endoplasmic reticulum. The amount of transported Ca2+ increased four times in the presence of 20 mM oxalate owing to the precipitation of calcium oxalate, which was detected inside the microsomal vesicles by electron microscopy. Ca2+ was released rapidly when the calcium ionophore, A-23187, was added. The Ca2+ concentration for the half-maximal activation of Ca2+ transport was about 1 microM. These results indicate that Ca2+ is translocated into the lumen of microsomes against a concentration gradient in a manner of the active transport. The microsomes showed Ca2+-dependent ATPase activity and were phosphorylated by the reaction with [gamma-32P]ATP in a similar Ca2+ dependence to that of transport rate. A 105-kDa phosphoprotein was identified by dodecyl sulfate polyacrylamide gel electrophoresis and was found to be sensitive to hydroxylamine. These properties of the phosphoprotein were the same as those of Ca2+ pump ATPase in the endoplasmic reticulum of other cells. These results suggest that the cytosolic Ca2+ is maintained at low levels by the microsomal uptake of Ca2+ by the action of the ATP-dependent Ca2+ pump or active transport system.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D019811:Hydroxylamine; D006898:Hydroxylamines / Q000494:pharmacology; D008854:Microscopy, Electron; D008861:Microsomes / Q000201:enzymology*; D008970:Molecular Weight; D010766:Phosphorylation; D013552:Swine; D013961:Thyroid Gland / Q000648:ultrastructure*; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology
chemical_list: D006898:Hydroxylamines; D014639:Vanadium; D019811:Hydroxylamine; D014638:Vanadates; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium