Comparative biochemistry of urea synthesis. 3. Activities of urea-cycle enzymes in various higher and lower vertebrates.

4. The ratio of potassium to sodium decreased significantly with advancing age in kidney and heart tissue. 5. Nine tissues were analysed from a further five LK and five HK aged ewes. The skeletal muscle, liver, spleen and kidney (expressed on a fatand blood-free basis) of the aged HK ewes tended to have higher concentrations of potassium (1.5, 2*2, 3-6 and 1-9% respectively) and lower concentrations of sodium (10-7, 12 3, 9 2 and 4 8% respectively) than those of the aged LK ewes. 6. Rumen epithelium from LK sheep had a significantly lower potassium concentration (19.5%) than that from HK animals. 7. The fat content of the skeletal muscle of aged LK sheep was significantly higher (17 g. of fat/kg. wet wt. or about 48%) than that of aged HK'sheep but this difference was not found in a group of lambs of the same breed.