ADIPOSE MONOACYLGLYCEROL-ACYL-COENZYME-A ACYLTRANSFERASE ACTIVITY IN THE WHITE-THROATED SPARROW (ZONOTRICHIA-ALBICOLLIS) - CHARACTERIZATIONAND FUNCTION IN A MIGRATORY BIRD

Although migrating birds use stored triacylglycerol as their primary f uel for flight, they must retain sufficient stores of omega 6 and omeg a 3 fatty acids to sustain reproduction after the spring migration. He patic monoacylglycerol:acyl-coenzyme A acyltransferase (EC 2.3.1.22) ( MGAT) activity is associated with physiological peri ods in which lipo lysis and beta-oxidation are prominent, and it may also play a role in the selective retention of certain essential fatty acids. Therefore, we characterized MGAT activity in adipose tissue from the white-throat ed sparrow (Zonotrichia albicollis), a migratory bird. MGAT specific a ctivity from adipose tissue and liver, respectively, was 22.2 +/- 7.27 and 0.79 +/- 0.35 nmol/min/mg of total particulate protein. Activity did not vary seasonally or between male and female birds. Specific act ivity increased 4.3 fold in the presence of 75 mu g of phosphatidylcho line and phosphatidylserine (1:1, w/w). MGAT acylated sn-1(3)-monooleo ylglycerol, sn-2-monooleylglycerol ether and sn-1(3)-monooleylglycerol ether at 7.5, 5.7 and 1.7%, respectively, of the rate observed with s n-2-monooleoylglycerol. An initial lag phase observed at low concentra tions of palmitoyl-CoA was corrected by adding 2 mM MgCl2, Mg(NO3)(2) or CaCl2, suggesting a requirement for divalent cations. MGAT acylated sn-2-monolinolenoylglycerol and sn-2-monolinoleoylglycerol in prefere nce to sn-a-monooleoylglycerol. Specificity of MGAT for sn-2-monoacylg lycerols and the probable enhanced affinity for sn-2-monoacylglycerols of specific acyl chains may allow selected omega 6 and omega 3 fatty acids to be retained within the adipocyte, while nonessential fatty ac ids are released for beta-oxidation in flight muscles.