NEURONAL NUCLEAR ACETYLTRANSFERASES INVOLVED IN THE SYNTHESIS OF PLATELET-ACTIVATING-FACTOR ARE LOCATED IN THE NUCLEAR-ENVELOPE AND SHOW DIFFERENTIAL LOSSES IN ACTIVITY

Neuronal nuclear fraction N-1 was isolated from cerebral cortices of 1 5-day-old rabbits, and nuclear subfractions prepared, in order to stud y the location of nuclear lyse platelet-activating factor (lyso-PAF) a cetyltransferase and alkylglycerophosphate (AGP) acetyltransferase, an d factors that affect the loss of these two nuclear activities. Subfra ctionation of prelabelled N-1 indicated that the nuclear envelope had the highest percentage of the radioactive acetylated products alkylace tylglycerophosphate (AAGP) and PAF, and the distribution of these phos pholipids reflected phospholipid distributions in the nuclear subfract ions. The majority (95%) of radioactive AAGP and PAF was also recovere d in Triton X-100 extracts of prelabelled nuclei, suggesting that thes e acetylated lipids are located in nuclear membranes rather than in th e nuclear matrix/chromatin. Of the nuclear subfractions, the envelope had the highest AGP and lyso-PAF acetyltransferase specific activities which were close to corresponding values seen in the parent N-1 fract ion. Thus the nuclear AGP and lyso-PAF acetyltransferases were princip ally localized to the nuclear membranes. Differentials in activity los s were seen for the two acetyltransferase activities. In the nuclear e nvelope fractions, the lyso-PAF acetyltransferase was the more suscept ible to oxidation reactions which could be reversed or blocked by the use of reducing agents. In preincubations, N-1 showed greater losses i n lyso-PAF acetyltransferase activity than in AGP acetyltransferase ac tivity, losses which were not attributable to oxidation. Addition of c ytosolic fraction S-3 to preincubations promoted losses for each acety ltransferase in N-1, and gave evidence for cytosolic and endogenous nu clear contributions to the activity loss. Addition of okadaic acid to the preincubations did not prevent the decline of either acetyltransfe rase in intact nuclei, but did diminish the loss of nuclear lyso-PAF a cetyltransferase activity promoted by S-3 addition, and also blocked t he loss of this acetyltransferase seen in preincubations of isolated n uclear envelopes. This suggests that nuclear lyso-PAF acetyltransferas e is susceptible to okadaic acid-sensitive nuclear and cytosolic prote in phosphatase activities, while AGP acetyltransferase may lose activi ty by the action of other phosphatases or by other mechanisms within t he nucleus.