Fatty acid specificities of microsomal acyltransferase esterifying positions-1 and −2 of acylglycerols in mammary glands from lactating rats

Abstract

The acyl specificities of several acyltransferases located in the microsomal fraction of lactating rat mammary gland have been investigated using palmitate and oleate as substrates along with CoA, ATP and Mg2+, bovine serum albumin and NaF. With either sn-glycerol 3-phosphate or dihydroxyacetone phosphate (plus NADPH) as acyl acceptor, phosphatidic acid containing palmitate preferentially esterified at position-2 and oleate at position-1 was the major product. Dihydroxyacetone phosphate and sn-glycerol 3-phosphate competitively inhibited each other's acylations, suggesting that a single enzyme might be responsible for both esterifications and oleate was the preferred substrate for the formation of acyldihydroxyacetone phosphate. The specificities of the acyl-CoA-1-monoacyl-sn-glycerol 3-phosphate and the acyl-CoA-2-monoacyl-sn-glycerol 3-phosphate acyltransferases were also studied. The specificities observed combined with the relative velocities of these reactions suggest that phosphatidic acid is formed in the mammary gland with the first acylation occurring at position-I favouring oleate followed by the second acylation at position-2 favouring palmitate. This is consistent with the unusual structure found in the triacylglycerols of rat milk. When a mouse liver microsomal fraction was used the opposite specificities were observed consistent with the structure of the triacylglycerols of mouse liver. The microsomal acylation of the monoacyl-sn-glycerol 3-phosphocholines was also investigated. Although no marked acyl specificity could be detected when the 2-monoacyl-sn-glycerol 3-phosphocholine was used as the acyl acceptor, both oleate and linoleate were esterified in preference to palmitate to the 1-monoacyl-sn-glycerol 3-phosphocholine.

Physiology and Biochemistry of Lactation

“…This circumstance does not arise in carcass fat (except pig lard), in which the 2-position usually acquires unsaturated fatty acids. Although the fatty acid specificities of the three enzymes that acylate these hydroxyl groups have not been exhaustively examined, there are signs that the complement of fatty acids is mainly determined by the specificities of the two enzymes acylating positions 1 and 2 of glycerol phosphate (Breach and Oils, 1975;Kinsella, 1976;Knudsen, 1977, 1979;S. Cooper and Grigor, 1980).…”

“…sn-GlyceroI3-phosphate is the primary acyl acceptor for fatty acids, being formed partly by the NADH-dependent reduction of dihydroxyacetone phosphate that arises from glycolysis, and partly from the ATP-dependent phosphorylation of free glycerol that arises from the hydrolysis of the plasma triacylglycerol supplying the fatty acids (see S. Cooper and Grigor, 1980;Oils, 1983). It is tempting to draw parallels with intestinal mucosa, where the dietary triacylglycerol is similarly disassembled, absorbed and reassembled.…”

Dietary Triacylglycerol Structure and Its Role in Infant Nutrition

“…In human milk, MCFA and 18:3(n-3) are higher at the TG sn-3 than at the sn-1 position, 22:6(n-3), 22:5(n-3), and adrenic acid 22:4(n-6) show preferential positioning at the sn-2 position, and 20:4(n-6) is relatively equally distributed on the sn-2,3 positions (2,4,19,21). The unusual positioning of fatty acids in human milk TG is the result of specificities of the mammary gland glycerol-3-phosphate (G-3-P) acyltransferase, which proceeds with acylation of an unsaturated fatty acid such as 18:1(n-9) at the sn-1 position of G-3-P followed by 16:0 at the sn-2 position (28,29). Early studies also described the TG structure of cow milk, which was the source of fat in infant formulas until the 1970s and is still used in some parts of the world.…”