Met-enkephalin immunoreactivity in blood platelets

Picogram amounts (50-150 pg/mg protein) of immunoreactive met-enkephalin material (met-enkephalin IR) were detected by radioimmunoassay in human, rat and rabbit platelets. Characterization of this material by thin-layer chromatography, gel filtration chromatography and high-pressure liquid chromatography indicated that it behaves identically with synthetic met-enkephalin. No high molecular weight met-enkephalin IR could be detected in the platelet extracts, even after trypsin hydrolysis, using two antisera which are able to recognize some of the putative met-enkephalin precursors present in the adrenal gland or striatum. In vitro, thrombin released platelet-met-enkephalin IR concomitantly with 5-hydroxytryptamine (5-HT), suggesting a common subcellular localization, i.e. the 5-HT storing organelles, for met-enkephalin IR and the amine. In vivo, platelet met-enkephalin IR in the Sprague-Dawley rat was affected neither by adrenalectomy nor by hypophysectomy. Thirteen-and 18-week-old spontaneous hypertensive rats (SHR) had lower platelet concentrations of met-enkephalin IR than age matched normotensive Wistar-Kyoto rats.