Liver microsomal mixed-function oxidases in response to dietary whole egg protein levels in rats

Relation between the activity of liver microsomal mixed-function oxidase system and dietary protein level was investigated in rats using purified whole egg protein, i.e. free from limiting amino acids. The animals were given either a diet containing 0, 5, 10, 20 or 40% of protein (experiment 1) or a diet containing 5, 10, 15 or 20% of protein (experiment 2) for 16 days. In experiment 2, half of the rats of each group were intraperitoneally injected sodium phenobarbital (PB) to induce the mixed-function oxidase system. The cytochrome P-450 content plateaued even at 5% level of dietary protein in experiment 1 and in the PB-untreated groups of experiment 2. However, it showed the highest value at 15% protein level in the PB-treated groups of experiment 2, indicating a shift of the response peak to a higher protein level due to an increase in protein requirement. Cytochrome P-450 reflected most specifically the dietary protein levels when the enzyme system was induced by PB. The 15% protein level, equivalent to 14.1 protein calories %, is a little higher than the optimal dietary level of whole egg protein ever obtained by usual nutritional indices.