Oscillations in rat liver cytosolic enzyme activities of the urea cycle.

Diurnal rhythms were studied in three rat liver enzymes of the urea cycle: arginase, arginosuccinate synthetase and arginosuccinase. In animals synchronized to a 12:12 h light-dark cycle these enzymes were determined at 8 different time points under three different feeding schedules: 24 h of fasting, ad libitum feeding and restricted feeding. Under the three experimental conditions maxima of enzyme activities occurred during the dark period. In all cases the maximum activity of arginosuccinase preceded the one of arginase and these in turn the one of arginosuccinate synthetase. On the other hand, the hepatic protein level was maximal during the light period and decreased to its lowest level during the dark period. The restriction of food between 08.00 h and 14.00 h induced an important phase shift of hepatic protein rhythm and arginosuccinase activity. Our results suggest that the diurnal rhythms of cytosolic enzymes of the urea cycle are not only dependent on the light-dark cycle, but also on the synchronizing and masking effect of food intake.