Effects of dietary casein and soy-protein on metabolic conversion of eicosapentaenoic acid to docosahexaenoic acid in the liver of rat.

The differential effects of dietary proteins on the metabolic process of eicosapentaenoic acid (EPA, 20:5n-3) via docosapentaenoic acid (DPA, 22:5n-3) to docosahexaenoic acid (DHA, 22:6n-3) were studied using parameters calculated from the proportion of n-3 polyenoic acid in liver phospholipid (PL) of rats. Rats were given casein or soy-protein isolate (SPI) diet containing 3% EPA with or without methionine (Met) supplementation for 3 weeks. The (22:5 + 22:6)/20:5 and 22:5/20:5 ratios of the PL fractions in the liver of rats given SPI were evidently elevated compared with those given casein. These ratios were also elevated when linoleic acid (LA, 18:2n-6) was added to the diet. There were no significant differences in the 22:6/22:5 ratio of the same PL fractions between the casein and SPI groups without the addition of LA, but the ratio was lowered by the addition of LA. It was found that these parameters for metabolic conversion were not affected by supplementation of Met or by a decrease in body weight. The above results suggest that the elongation step from EPA to DPA is affected by the type of protein, i.e., it is accelerated by SPI and is not affected by the presence of LA, while the metabolic process from DPA to DHA is not affected by the type of proteins without the addition of LA but is suppressed when LA is added.