BIOSYNTHESIS OF PLASMALOGENS BY THE MICROSOMAL FRACTION OF FISCHER R-3259 SARCOMA - INFLUENCE OF SPECIFIC 2-ACYL CHAINS ON THE DESATURATION OF 1-ALKYL-2-ACYL-SN-GLYCERO-3-PHOSPHOETHANOLAMINE

In the Fischer R-3259 sarcoma, ethanolamine plasmalogens (1 -alk- l'-enyl-2-acyl-.sn-glycero-3-phosphoethanolamine) are synthesized from 1-alkyl-2-acyl-sn-glyc-ero-3-phosphoethanolamine (l-alkyl-2-acyl-GPE) by a microsomal desaturase that inserts a A1 double bond in the alkyl chain. In the present study, a series of l-[l-14C]hexadecyl-2-acyl-GPE substrates containing specific acyl groups ranging from C2:0 to C20:4 at the 2 position were prepared and tested as substrates for the microsomal Δ1-alkyl desaturase. The microsomal preparations contained an acyl hydrolase that removed the C2;0, C4:0, and C7:0 acyl groups from the 2 position. By inhibiting the hydrolase with diisopropyl fluorophosphate, it was possible to test conversion of the unaltered substrates to plasmalogens. The alkyl desaturase exhibited little discrimination among the specific acyl derivatives tested. The highest rate of desaturation was obtained with 1-[1-14C]-hexadecyl-2-acyl-GPE synthesized in situ in the microsomes via acylation of l-[l-14C]hexadecyl-GPE; this rate was threefold that observed with exogenously acylated substrates. The l-[l-l4C]hexadecyl-2-acyl-GPE synthesized in situ contained highly unsaturated acyl groups; no selectivity of the desaturase for specific acyl chains was detected when the different molecular species of l-[l-14C]alkyl-2-acyl-GPE and l-[l-14C]alk-1′-enyl-2-acyl-GPE were compared. The short-chain substrates, being more hydrophilic, mimicked the chromatographic behavior of 1-alkyl-GPE, yet they did not resemble the lyso compound in its higher conversion to plasmalogens. Thus, despite their similar Rf values, the packing of the short-chain acyl homologues in the membrane may be quite different from that of the lyso compound. Binding of l-hexadecyl-2-acyl-GPE and 1-hexadecyl-GPE to microsomal membranes was similar. Detergents stimulated the desaturation of exogenously added l-hexadecyl-2-acyl-GPE, but the level of activity remained at only one-third that obtained by using 1-hexadecyl-GPE as the precursor. The results suggest that the acylation step positions the substrate in the membrane so that it becomes more accessible to the alkyl desaturase. © 1979, American Chemical Society. All rights reserved.