Adaptive enzyme changes in liver and muscle of rats during protein depletion and refeeding.

I . Young rats were kept on a 6 % casein diet for 4 7 weeks and then either killed or returned to a stock diet. Control rats were given a stock diet. 2. Argininosuccinate lyase (EC 4.3.2. I , L-argininosuccinate arginine-lyase) was assayed in liver, and amino acid activating enzymes were assayed in liver and muscle of rats after depletion and while they were refed on the stock diet. Levels of enzymes were compared with those in the control rats. 3. Amino acid activating enzymes were markedly increased in liver at the end of the depletion period, but returned almost to normal levels after 4-6 days of refeeding. In muscle the enzyme levels were not decreased after depletion, but began to rise after 4 days of refeeding. 4. Argininosuccinate lyase activity in depleted female rats was about half that in control rats and rose again slowly on refeeding. Male rats were hardly affected by depletion. 5 . It is suggested that these enzyme changes play a part in the adaptation of the rat to a reduced protein intake.