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 p arameters calculated from the proportion of n-3 polyenoic acid in live r phospholipid (PL) of rats. Rats were given casein or soy-protein iso late (SPI) diet containing 3% EPA with or without methionine (Met) sup plementation for 3 weeks. The (22:5 + 22 : 6)/20 : 5 and 22:5/20:5 rat ios of the PL fractions in the liver of rats given SPI were evidently elevated compared with those given casein. These ratios were also elev ated when linoleic acid (LA, 18:2n-6) was added to the diet. There wer e no significant differences in the 22:6/22:5 ratio of the same PL fra ctions between the casein and SPI groups without the addition of LA, b ut the ratio was lowered by the addition of LA. It was found that thes e parameters for metabolic conversion were not affected by supplementa tion 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 p rotein, i.e., it is accelerated by SPI and is not affected by the pres ence of LA, while the metabolic process from DPA to DHA is not affecte d by the type of proteins without the addition of LA but is suppressed when LA is added.