ACTIVITIES OF ENZYMES IN PLATELET-ACTIVATING-FACTOR BIOSYNTHETIC PATHWAYS IN THE GERBIL MODEL OF CEREBRAL-ISCHEMIA

The activities of enzymes in platelet activating factor (PAF) biosynth etic pathways were analyzed in hippocampal and cerebral cortical regio ns of normal and ischemic gerbil brain to assess changes in enzyme act ivities and potential modulators that could explain the accentuated pr oduction of PAF seen in ischemia. Global forebrain ischemia was produc ed by bilateral carotid artery ligation, and the effectiveness of the ligation was shown by free fatty acid release and AT depletion. Specif ic activities of 1-alkyl-2-acetyl-sn-glycerol (AAG) choline phosphotra nsferase, 1-alkyl-sn-glycero-3-phosphate (AGP) acetyl transferase, and 1-alkyl-sn-glycero-3-phosphocholine (lyso PAF) acetyl transferase in tissue homogenates were in the ratio 4:1:0.1, respectively. Sham-opera ted and ischemic or ischemic-reperfused tissues showed similar activit ies for individual enzymes, indicating that enzyme levels or activatio n states did not change in ischemic or reperfused tissues. However, sm all metabolites (relevant to ischemia) added to the in vitro assays di d modify enzyme activities. Physiological concentrations of MgATP seve rely inhibited AGP acetyl transferase activity, and this resulted in t he ratio of AGP acyl transferase to AGP acetyl transferase activities changing from 48:1 in the presence of 2.5 mM MgATP to 6:1 in the absen ce of MgATP. This suggests that falling ATP levels in cerebral ischemi a may promote the de novo pathway of PAF biosynthesis by releasing inh ibition of AGP-acetyl transferase. Lyse PAF acetyl transferase was muc h less active than AGP acetyl transferase and was also inhibited by Mg ATP. AAG choline phosphotransferase was not inhibited by MgATP but was inhibited by calcium. However the superior specific activity of the c holine phosphotransferase in comparison with the AGP acetyl transferas e suggested that the lowered choline phosphotransferase activity in th e presence of rising intracellular calcium would not seriously comprom ise the synthesis of PAF by the de novo route. Both acetyl transferase enzymes were also inhibited by oleoyl CoA.