Acyl specificity in glyceride synthesis by lactating rat mammary gland

We have investigated the possibility that the nonrandom association of fatty acids in rat milk triglycerides results from specificity of the acyl transferases in the glycerolphosphate pathway. Subcellular fractionation of lactating rat mammary gland revealed that the microsomal fraction was the most active in acylation of 3-sn-U-¹⁴C] glycerolphosphate with various acyl-CoA's. The major products were diacylglycerolphosphate and diglyceride; no monoacylglycerolphosphate was detected. Maximum rate of acylation occurred at or below the critical micelle concentration for each acyl-CoA, indicating that only the monomeric substrate molecules were acceptable by the enzyme system. The observed acyl specificity, 16∶0>18∶0?14∶0>12∶0>10∶0>8∶0 is consistent with the concept that, in general, milk triglycerides are synthesized by insertion of a short or medium chain fatty acid into a long chain diglyceride.