Differences in the interconversion between 20- and 22-carbon (n - 3) and (n - 6) polyunsaturated fatty acids in rat liver.
When male weanling rats were fed diets containing either 5% corn oil or a diet in which half of the corn oil was replaced by fish oil, the 20:5(n - 3) in liver choline and ethanolamine phosphoglycerides, not only partially replaced arachidonate but also paired with palmitic and stearic acids in the same molar ratio as did arachidonate. The 22:5(n - 3)/22:6(n - 3) ratio in the liver phospholipids of corn oil fed rats was similar to that found when the esterified levels of these two acids were increased 5-fold by feeding fish oil. Moreover, the pairing of both 22:5(n - 3) and 22:6(n - 3) with palmitic and stearic acids, on a molar ratio basis, was relatively independent of the total amount of esterified 22:5(n - 3) and 22:6(n - 3). When (3-14C)-labeled 22:4(n - 6) was injected into rats raised on a chow diet or incubated with hepatocytes from these animals, its primary metabolic fate was retroconversion to arachidonate followed by esterification. Conversely, [3-14C]22:5(n - 3) was a poorer substrate for retroconversion with a larger amount being esterified directly into phospholipids and, in addition, this acid served as a precursor for 22:6(n - 3). The enhanced metabolism of both [3-14C]22:4(n - 6) to 22:5(n - 6) and of [3-14C]22:5(n - 3) to 22:6(n - 3) in animals raised on a diet devoid of fat or in their hepatocytes may possibly be due to elevated 6-desaturase activity and/or the level of this enzyme or enzymes. This hypothesis is based on studies showing that the synthesis of 22:6(n - 3) proceeds via a pathway independent of a 4-desaturase but requires the use of a 6-desaturase at two steps (Voss, A., Reinhart, M., Sankarappa, S. and Sprecher, H. (1991) J. Biol. Chem. 266, 19995-20000).
Voss
A
A
Department of Medical Biochemistry, Ohio State University, Columbus 43210.
Reinhart
M
M
Sprecher
H
H
D000818:Animals; D002250:Carbon Radioisotopes; D002478:Cells, Cultured / Q000378:metabolism; D003314:Corn Oil / Q000008:administration & dosage; D004032:Diet; D015525:Fatty Acids, Omega-3 / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology; D043371:Fatty Acids, Omega-6; D005231:Fatty Acids, Unsaturated / Q000008:administration & dosage / Q000378:metabolism* / Q000494:pharmacology; D005395:Fish Oils / Q000008:administration & dosage; D008099:Liver / Q000378:metabolism*; D008297:Male; D010713:Phosphatidylcholines / Q000032:analysis; D010714:Phosphatidylethanolamines / Q000032:analysis; D010743:Phospholipids / Q000096:biosynthesis; D051381:Rats; D011919:Rats, Inbred Strains
D002250:Carbon Radioisotopes; D015525:Fatty Acids, Omega-3; D043371:Fatty Acids, Omega-6; D005231:Fatty Acids, Unsaturated; D005395:Fish Oils; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D010743:Phospholipids; D003314:Corn Oil