Incorporation of [14C]leucine into protein of perfused liver of rats fed diets containing casein or lactalbumin.

The livers of rats (190-210 g) fed casein (a phosphoprotein) or lactalbumin (non-phosphoprotein) diets (200 g protein/kg diet) were perfused for 20, 40, or 60 minutes with 5 microCi of [14C]leucine. We determined the amount of radioactivity in the protein and non-protein (free amino acid) fractions of the liver and perfusate supernatant. The wet weight, total protein and protein: DNA ratio of the livers was also determined. The livers of the group fed casein were significantly lighter and contained less protein than the lactalbumin-fed group. However, more [14C]leucine was incorporated into protein of the liver plus perfusate supernatant of the casein-fed group than the lactalbumin group. We concluded that protein metabolism in the liver of rats appears to be increased when the rats were fed a diet containing phosphoprotein as the sole source of protein.