CHARACTERIZATION OF LYSOPHOSPHOLIPID METABOLIZING ENZYMES IN HUMAN BRAIN

Lysophospholipids are generated during the turnover and breakdown of m embrane phospholipids. We have identified and partially characterized three enzymes involved in the metabolism of lysophospholipids in human brain, namely, lysophospholipase, lysophospholipid:acyl-CoA acyltrans ferase (acyltransferase), and lysophospholipid:lysophospholipid transa cylase (transacylase). Each enzyme displayed comparable levels of acti vity in biopsied and autopsied human brain, although in all cases the activity was somewhat lower in human than that in rat brain. All three enzymes were localized predominantly in the particulate fraction, wit h lysophospholipase possessing the greatest activity followed by acylt ransferase and transacylase. Lysophosphatidylcholine possessed a K-m i n the micromolar range for lysophospholipase and transacylase, and in the millimolar range for acyltransferase, whereas arachidonyl-CoA disp layed a K-m in the micromolar range for acyltransferase. The three enz ymes differed in their pH optima, with lysophospholipase being most ac tive at pH 8.0, transacylase at pH 7.5, and acyltransferase at pH 6.0. Both bromophenacyl bromide and N-ethylmaleimide inhibited lysophospho lipase activity and, to a lesser extent, that of acyltransferase and t ransacylase. None of the enzyme activities were affected by the presen ce of dithiothreitol or EDTA, although particulate lysophospholipase w as activated approximately twofold by the addition of 5 mM MgCl2 or Ca Cl2 but not KCl. Transacylating activity was stimulated by CoA, the EC (50) of activation being 6.8 mu M. Acyltransferase displayed an approx imately threefold preference for arachidonyl-CoA over palmitoyl-CoA, w hereas the acylation rate of different lysophospholipids was in the or der lysophosphatidylinositol > 1-palmitoyl lysophosphatidylcholine > 1 -oleoyl lysophosphatidylcholine much greater than lysophosphatidylseri ne > lysophosphatidylethanolamine. This, and the preference of human b rain phospholipase A(2) for phosphatidylinositol, suggests that this p hospholipid may possess a higher turnover rate than the other phosphol ipid classes examined. Human brain homogenates also possessed the abil ity to transfer fatty acid from lysophosphatidylcholine to lysophospha tidylethanolamine. In addition, we also present evidence that diacylgl ycerophospholipids can act as acyl donors for the transacylation of ly sophospholipids. We have therefore demonstrated the presence of, and p artially characterized, three enzymes that are involved in the metabol ism of lysophospholipids in human brain. Our results suggest that lyso phospholipase may be the major route by which lysophospholipids are re moved from the cell membrane in human brain. However, all three enzyme s likely play an important role in the remodeling of membrane composit ion and thereby contribute to the overall functioning of membrane-asso ciated processes.