Different metabolic rates for arachidonoyl molecular species of ethanolamine glycerophospholipids in rat brain.

The ethanolamine glycerophospholipids (EGP) contain most of the arachidonate (20:4, n-6) and adrenate (22:4, n-6), potential precursors of biologically potent prostaglandins and related compounds. Much better methods utilizing high performance liquid chromatography (HPLC) techniques are now available for the study of the molecular species of all three classes, namely diacyl, alkenylacyl (plasmalogen), and alkylacyl. Different molecular species may have different functions. This possibility was studied by examining the rates of incorporation of [3H]arachidonic acid into the three major molecular species of each of the three classes of ethanolamine glycerophospholipids. After the intracerebral injection of [3H]20:4 into rat brain, it was rapidly converted to 22:4(n-6). Of the total radioactivity, 10-20% was located in 22:4 in alkenylacyl and diacyl-GPE. In the alkylacyl-GPE, labeled 22:4 was preferentially incorporated and accounted for 50-60% of the total radioactivity. The primary arachidonoyl molecular species of alkenylacyl, alkylacyl, and diacyl-GPE were the 18:1-20:4, 16:0-20:4, and 18:0-20:4 species. The alkylacyl class contained almost equal proportions of these three molecular species. On the other hand, the 20:4 in alkenylacyl and diacyl classes was combined largely with 18:0 groups at the sn-1 position. In particular, the 18:0-20:4 species comprised about 80% of arachidonoyl molecular species of the diacyl class. In all three classes, the highest specific radioactivities were found in the 18:1-20:4 species, whereas the 18:0-20:4 species had the lowest specific radioactivity. Over the period 60 min-24 hr, the diacyl 18:0-20:4 and all three arachidonoyl molecular species of the alkenylacyl class increased in specific radioactivity more rapidly than the other arachidonoyl molecular species.