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. We wish to express our appreciation to Dr A. T. Phillipson, Head of the Physiology Department, for his advice and very helpful discussions. We would also like to thank

Glucose-6-phosphate dehydrogenase from human erythrocytes shows successive aggregationdissociation equilibria. The effects of salt concentration and pH show that a molecule of 210,000 molecular weight and ~2 0 ,~ of 9.0 S dissociated to a half-molecule of 105,000 molecular weight and S Z O ,~ of 5.6 S. Evidence for this being a discrete dissociation was provided by molecular weight determinations, relation between sedimentation coefficients and molecular weights, and the occurrence of skew patterns a t a particular degree of dissociation.Some aggregation of the molecule to give apparent molecular weights above 210,000 occurred a t low s d t concentrations in the pH range 6-7.Reaction with maleic anhydride gave a species of molecular weight 53,000 and ~2 0 ,~ S,indicating that in the native protein the predominant molecular species (210,000) was a tetramer.The nature of the dissociations indicate the dissimilarities between the subunit contacts in the tetrameric molecule. Optimum conditions for the enzyme assay suggest that the dimer contains the essential requirements for the catalytic function of the protein, and the implications of these findings are discussed.

The synthesis and intracellular transport of the mitochondrial matrix enzymes ornithine transcarbamylase (carbamoylphosphate: L-ornithine carbamoyltransferase, EC 2.1.3.3.) and carbamoyl-phosphate synthetase (ammonia) I [carbon-dioxide:ammonia ligase (ADP-forming, carbamate-phosphorylating), EC 6.3.4.16] were studied in isolated rat hepatocytes. In pulse experiments at 37C, the larger precursors of the two enzymes appeared in the cytosol ofthe liver cells, where radioactivity levels of the precursors reached a plateau in 10-20 min after the pulse. The pulse-labeled mature enzymes appeared in the particulate fraction (containing mitochondria) after a time lag and increased almost linearly with time up to 40 min. The specific radioactivities of the precursors in the cytosol were much higher than those of the mature enzymes in the particulate fraction. In pulse-chase experiments, the labeled precursors disappeared from the cytosol with estimated half-lives of about 1-2 min. These results indicate that ornithine transcarbamylase and carbamoyl-phosphate synthetase I are initially synthesized as larger precursors and exist in a cytosolic pool from which they are transported into mitochondria and processed there to the mature enzymes concomitandy with or immediately after transport. Although the rates of synthesis, transport, and processing were decreased about 3-fold at 25'C (as compared to incubation at 37C), the pool size of the precursors in the cytosol were somewhat larger at this temperature.

No abstract

Rats given a lethal dose (LD99.9) of ammonium acetate (10.8 mmol/kg of body weight) were protected to the extent of 85 and 76% when previously injected with N-carbamoyl glutamate or L-arginine, respectively, at a level of 4 mmol/kg of body weight. At a dose of 1 mmol/kg of body weight, L-arginine protected 24%, while N-carbamoyl-L-glutamate protected 61 % of the animals. When a combination of N-carbamoyl-L-glutamate plus L-arginine (1 mmol each per kg of body weight) was injected, 100% of the rats were protected. The efficacy of N-carbamoyl-L-glutamate is related to its role as an activator of mitochondrial carbamoyl phosphate synthetase (EC 2.7.2.5) and its resistance to hydrolysis by tissue acylaminoacid acylase. N-Acetyl-IL-glutamate, the naturally occurring and most effective activator of mitochondrial carbamoyl phosphate synthetase, was relatively ineffective in protection against lethal dose of ammonium acetate, because of its ready hydrolysis by acylaminoacid acylase.Thq findings reported provide a rational basis for the use of N-carbamoyl-L-glutamate plus L-arginine in the prevention and treatment of hyperammonemia in clinical conditions of liver disease and parental infusion of amino acids, and in feeding of urea supplements to ruminants.