title: Ionophorous properties of the 13 000-Da fragment from sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase.
abstract: The 25 000-Da tryptic fragment from rabbit muscle sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase was subjected to cyanogen bromide digestion, and the four fragments isolated. Only the 13 000-Da fragment induced ionophorous activity in planar thin lipid membranes made with 5:1 (w/w) phosphatidylcholine/cholesterol in decane. The membranes became cation selective, with a selectivity sequence among divalent of Mn2+ greater than Ca2+ greater than Ba2+ greater than Sr2+ greater than Mg2+. This is different from that of the 25 000-Da fragment (A.E. Shamoo, 1978, J. Memb. Biol. 43, 227-242), it's 'parent' 55 000-Da fragment, and the intact enzyme, all of which have the same selectivity sequence. The inhibitory effects of Hg2+, Cd2+ and Zn2+ were also examined. All were inhibitory, with Zn2+ being the most effective of these. The heavy-metal-induced inhibition of Ca2+ conductance could be reversed by selective chelation of the heavy metals by EDTA. From changes in the selectivity as well as changes in heavy-metal-induced inhibition behavior, we conclude that the ion transport site of the 13 000-Da fragment may not be the same site as that of the parent fragment. It is either a different site altogether or has been physically modified by peptide cleavage.
mesh_terms: D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D002104:Cadmium / Q000494:pharmacology; D019256:Cadmium Chloride; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002712:Chlorides; D007476:Ionophores; D007700:Kinetics; D008627:Mercuric Chloride; D008628:Mercury / Q000494:pharmacology; D008970:Molecular Weight; D009132:Muscles / Q000201:enzymology; D010446:Peptide Fragments / Q000378:metabolism; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D014357:Trypsin; D015032:Zinc / Q000494:pharmacology; D017967:Zinc Compounds
chemical_list: D002712:Chlorides; D007476:Ionophores; D010446:Peptide Fragments; D017967:Zinc Compounds; D002104:Cadmium; D008627:Mercuric Chloride; C016837:zinc chloride; D014357:Trypsin; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008628:Mercury; D015032:Zinc; D019256:Cadmium Chloride