I . Measurements were made, at 6 h intervals, of urinary nitrogen output and of the activity of some hepatic enzymes in the rat during adaptation from one level of dietary protein to another. The enzymes measured were arginase (EC 3.5.3. I 3. When the dietary protein content was reduced from 135 to 45 g caseinlkg, the urinary N output and the activities of the hepatic enzymes reached their new steady-state levels in 30 h. The reverse adaptation, from 45 to 135 g casein/kg, was also complete in 30 h.4. The rate of change of enzyme activity and the final activity as percentage of initial activity were very similar for all six enzymes, suggesting a common control mechanism. The calculated half-lives of the enzymes were of the order of 7 h, which is very much shorter than those found by previous workers.5 . There was no simple relationship between the activity of the urea cycle enzymes and the amount of N excreted. When an equal amount of gelatin was substituted for casein the N output was doubled but there was no change in the activity of the liver enzymes. 6. The results suggest that the activity of the urea cycle enzymes depends in part on the amount of N available for excretion after the demands for synthesis have been met. The enzymes, however, appear to be present in excess so that an increased N load was not necessarily accompanied by an increase in enzyme activity. This paper is concerned with the mechanism by which urinary nitrogen output is adjusted to match the intake, and in particular with the part played in this process by adaptive changes in hepatic enzymes. Schimke (1962) was the first to show that in the rat the activity of the urea cycle enzymes, as measured in vitro, is related to the level of dietary protein intake. This was fully confirmed by later work (Schimke, 1964; Harper, 1965; Stephen, 1968). Schimke (1964) also found a rise in the activity of the urea cycle enzymes in starvation, when there is an increase in urinary N output as a result of the breakdown of tissue protein. McLean & Gurney (1963) showed that the urea cycle enzymes are sensitive to adrenalectomy and to the administration of cortisone and growth hormone.Similar adaptive changes are shown by enzymes in the liver which make amino groups available for entry into the urea cycle ; alanine aminotransferase (EC 2.6. I . 2)

-Five total mixed rations prepared from finger millet (Eleusine Coracana) straw as a roughage (48%) and mixed concentrate (52%), supplemented with a 1% isoacid mixture (i-C 4 , i-C 5 , C 5 and phenylacetic acid in equal proportions) or oil (groundnut oil, 5% more than the control) or urea (5% more nitrogen than the control), and protein (groundnut cake, 5% more nitrogen than the control) were given in a Latin square experiment to sheep. Enzymatic activities were estimated for urease, cellulase, protease, amylase, and lipase in various fractions of rumen fluid on the one hand and rumen microbial biomass on the other hand. Rumen samples were taken 3-4 hours after feeding and mixed rumen bacteria were separated as a strained rumen fluid without protozoa (SRFWP), cell free rumen fluid (CFRF) and enzymes associated with the bacteria cell (EABC). Samples of SRFWP and EABC contained higher enzyme activities than CFRF. Depending on the type of enzymes in each fraction, some significant coefficient of determination (r 2 ) was seen. These values showed very close cooperative action between proteolytic and amylolytic enzymes under the experimental conditions, or perhaps the presence of some species of bacteria with both activities. Lipolytic bacteria are completely specialized for lipase production only (P < 0.05). The results showed oil, isoacid and crude protein enhanced microbial production (P < 0.05) and this can change the pattern of enzymes in the rumen of sheep.isoacid / urease / cellulase / protease / amylase / lipase / sheep / rumen Reprod. Nutr. Dev. 41 (2001) 513-529 513