Adipose monoacylglycerol: Acyl‐coenzyme a acyltransferase activity in the white‐throated sparrow (Zonotrichia albicollis): Characterization and function in a migratory bird

Although migrating birds use stored triacylglycerol as their primary fuel for flight, they must retain sufficient stores of omega 6 and omega 3 fatty acids to sustain reproduction after the spring migration. Hepatic monoacylglycerol:acyl-coenzyme A acyltransferase (EC 2.3.1.22) (MGAT) activity is associated with physiological periods in which lipolysis 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-throated sparrow (Zonotrichia albicollis), a migratory bird. MGAT specific activity 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 activity increased 4.3-fold in the presence of 75 micrograms of phosphatidylcholine and phosphatidylserine (1:1, w/w). MGAT acylated sn-1(3)-monooleoylglycerol, sn-2-monooleylglycerol ether and sn-1(3)-monooleylglycerol ether at 7.5, 5.7 and 1.7%, respectively, of the rate observed with sn-2-monooleoylglycerol. An initial lag phase observed at low concentrations 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 preference to sn-2-monooleoylglycerol. Specificity of MGAT for sn-2-monoacylglycerols and the probable enhanced affinity fo 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 acids are released for beta-oxidation in flight muscles.