Journal of Lipid Research, Vol 17, 647-656, Copyright © 1976 by Lipid Research, Inc.
Acyl specificity in triglyceride synthesis by lactating rat mammary gland
CY Lin, S Abraham and S Smith
We have studied the specificity of the acyl-CoA:diglyceride acyltransferase
reaction in lactating rat mammary gland to provide a rational explanation
at the enzyme level for the nonrandom distribution of fatty acids in milk
fat triglycerides. Acyl-CoA:diglyceride acyltransferase activity was
measured using various diglyceride and radioactive acyl-CoA substrates;
products were identified as triglycerides by thin-layer and gas-liquid
chromatography. Most of the enzymatic activity was located in the
microsomal fraction and showed a broad specificity for the acyl donors
tested C10, C12, C14, C16, C18, and C18:1 CoA esters). The acyltransferase
activity was highly specific for sn-1,2-diglyceride enantiomers; rac-1,3-
and sn-2,3-diglycerides were relatively inactive. The acyl-CoA specificity
was not affected by the type of 1,2-diglyceride acceptor offered, although
dilaurin was the best acceptor and sn-1,2-dilaurin greater than
sn-1,2-dimyristin greater than sn-1,2-dipalmitin greater than
sn-1,2-distearin. We have previously shown that in the microsomal fraction
from lactating rat mammary gland, the acyltransferase activities concerned
with the conversion of sn-glycero-3-phosphate to diacylglycerophosphate
show a very marked specificity for long chain acyl-CoA's. Therefore, we
conclude that the predominant localization of long chain fatty acids in the
1 and 2 positions, and of shorter chain fatty acids in the 3 position of
the glycerol backbone, results at least in part from the specificities of
the mammary gland acyltransferases.
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