HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Adunyah, S. E. Articles by Dean, W. L. Search for Related Content PubMed PubMed Citation Articles by Adunyah, S. E. Articles by Dean, W. L. Social Bookmarking                      What's this?

J. Biol. Chem., Vol. 261, Issue 7, 3122-3127, Mar, 1986

Ca2+ transport in human platelet membranes. Kinetics of active transport and passive release

SE Adunyah and WL Dean

Active Ca2+ transport and passive release were characterized in crude and purified human platelet membranes to facilitate comparison with skeletal muscle sarcoplasmic reticulum. Endoplasmic reticulum markers were enriched from 3- to 14-fold in the purified membranes, while surface membrane antigens were reduced 4-fold and mitochondrial contamination was completely eliminated. The pH optimum for active Ca2+ transport in platelet membranes was 7.6, and the optimum for Ca2+- ATPase activity ranged from 7.6 to 8.0. Upon addition of MgATP there was a burst in active Ca2+ transport activity. In the absence of phosphate, steady state was reached within 20 s; added phosphate promoted continued uptake for greater than 1 h. The maximum pump stoichiometry was 2.0 Ca2+/ATP. The Ca2+ ionophore A23187 caused rapid release of 90% of the sequestered Ca2+ in the presence of phosphate. The dependence of Ca2+ transport on MgATP was biphasic with apparent Km values of 0.6 mM and 9.5 microM. Kinetic measurements with varied external Ca2+ yielded a single Km of 0.1 microM. Mg2+ stimulated Ca2+ transport and Ca2+-ATPase activities. Results with crude and purified membranes were similar, and comparison with the Ca2+ pump from sarcoplasmic reticulum revealed nearly identical enzymatic properties. In contrast to the results of comparing active Ca2+ transport, the characteristics of Ca2+ release from platelet membranes were quite different from those of sarcoplasmic reticulum. External Ca2+ did not promote release of sequestered Ca2+ from platelet membranes in contrast to sarcoplasmic reticulum. In addition, spontaneous release of Ca2+ from platelet membranes did not occur after ATP depletion. Inositol trisphosphate induced rapid partial release of Ca2+ from platelet membranes but had no effect on sarcoplasmic reticulum under identical conditions. Thus active Ca2+ transport is quite similar in internal membranes of platelet and skeletal muscle, but the mechanism of Ca2+ release appears to be entirely different.
CiteULike    Complore    Connotea    Del.icio.us    Digg    Reddit    Technorati    What's this?