Acyltransferase activities in the yolk sac membrane of the chick embryo

The activities of some enzymes of glycerolipid synthesis and fatty acid oxi dation were measured in subcellular fractions of the yolk sac membrane (YSM ), an extra-embryonic tissue that mediates the transfer of lipid from the y olk to the circulation of the chick embryo. The activities of monoacylglyce rol acyltransferase and carnitine palmitoyl transferase-1 in the YSM (respe ctively, 284.8 +/- 13.2 nmol/min/mg microsomal protein and 145.6 +/- 9.1 nm ol/min/mg mitochondrial protein; mean +/- SE; n = 4) at day 12 of developme nt appear to be the highest yet reported for any animal tissue. Also, the c arnitine palmitoyl transferase-1 of the YSM was very insensitive to inhibit ion by malonyl CoA. The maximal activities of glycerol-3-phosphate acyltran sferase and diacylglycerol acyltransferase in the YSM (respectively, 26.7 /- 2.2 and 36.1 +/- 2.1 nmol/min/mg microsomal protein) were also high comp ared with the reported values for various animal tissues. The very high enz ymic capacity for glycerolipid synthesis supports the hypothesis that the y olk-derived lipids are subjected to hydrolysis followed by reesterification during transit across the YSM. The monoacylglycerol path way appears to be the main route for glycerolipid resynthesis in the YSM. The results also s uggest that the YSM has the capacity to perform simultaneously beta-oxidati on at a high rate in order to provide energy for the lipid transfer process .