Serine base exchange enzyme in porcine lyophilised platelets: enzyme properties and modulation by AIF(4)(-) and different types of heparin

Phosphatidylserine is one of the PKC modulators and thus it may play an imp ortant role in signal transduction. Regulation of the synthesis of this pho spholipid is not yet clarified. The contrasting reports are possibly relate d to the existence of different enzymes which, in mammalian tissues, cataly se the exchange between free serine and the nitrogen base of a membrane pho spholipid. This study demonstrates that serine base exchange reactions of c ommercially available lyophilised porcine platelets exhibit similar pH opti ma, temperature and Ca2+ dependence as observed in fresh tissues. Analysis of fatty acids composition of the three phospholipid classes involved in ba se exchange reactions also demonstrated a similarity with fresh platelets. Serine and ethanolamine base exchange enzyme activities were assayed in par allel in platelet lysate subjected to preincubation at various temperatures (30-60 degrees C). When dithioerithrol was omitted from the incubation med ium, the two base exchange reactions were inhibited with a similar temperat ure-dependent pattern. Addition of the reducing agent enhanced the sensitiv ity to preincubation only for the serine base exchange reaction which was i nhibited by 80% after preincubation at 45 degrees C. With respect to its regulation, porcine platelet serine base exchange enzym e(s) was inhibited by fluoroalluminate, a widely used G-protein activator, and stimulated by unfractionated heparin. Low mol. wt. heparin did not infl uence enzyme activity. Unfractionated heparin greatly stimulated SBEE activ ity assayed at pH 7.4, a pH value far from the optimal pH.