Substrate specificities of neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor: Differences in the use of 1-alkyl and 1-acyl lysophospholipid acceptors

The selectivity of alkylglycerophosphate (AGP) acetyltransferase and lyso-platelet-activating factor (lyso-PAF) acetyltransferase was studied in neuronal nuclei isolated from cerebral cortices of 15-day-old rabbits. Specifically, I-alkyl and l-acyl analogues were compared as accepters in these acetylation reactions. A number of observations supported one nuclear activity in the acetylation of AGP and lyse-PA. Lyse-PA was a competitive substrate for AGP, K-m values for AGP and lyse-PA were similar, as were acetylation rates measured at individual AGP or lyse-PA concentrations, and the acetylation of both substrates was unaffected by preincubations with protein phosphatase 1 (PP-1). In contrast, there were a number of differences seen in the acetylation of lyso-PAF and lyse-PC. The kinetics for lyse-PC acetylation (as a function of lyse-PC concentration) were not hyperbolic, and lyse-PC was not a competitive substrate for the acetylation of lyso-PAF. Unlike acetylation rates with lyso-PAF, lyse-PC acetylation was not reduced by preincubations with PP-1, and was less susceptible to inhibition particularly at high levels of free fatty acid. In addition, rates of acetylation of lyse-PC were selectively increased by the presence of lyse-PA. When neuronal nuclear envelope fractions (NE) were prepared from N-l, the specific acetylation activity with lyso-PAF was significantly lower in NE, while the activities for lyse-PC were comparable in NE and the parent N-l fraction. The results with the acetylation of lyse-PC and lyso-PAF suggest that the lyse-PC acetyltransferase may be in a uniquely sequestered state within the neuronal nucleus. This could explain the smaller inhibition of lyse-PC acetylation by free fatty acid, the maintenance of lyse-PC acetylation during PP-1 preincubations, the non-hyperbolic response to lyse-PC concentrations and the selective preservation of lyse-PC acetylation during NE isolation. This protected status could result from a more internal location for this acetyltransferase within the membranes of the nuclear envelope, or possibly an association of the enzyme with the nuclear matrix that is disrupted with the exposure of N-l to lyse-PA. (C) 1998 Elsevier Science B.V.