In a study, cats were fasted overnight, then given a single meal complete in all amino acids except arginine. Within two hours, blood ammonia levels increased from a normal level of 18 µg/L to 140 µg/L, and the cats showed the clinical symptoms of ammonia toxicity. A control group fed a complete amino acid diet, or an amino acid diet in which ornithine replaced arginine, showed no unusual clinical symptoms. --- **What was the role of fasting in the experiment?** - ○ to lower blood glucose levels - ○ to help diagnose arginase deficiency disease - ○ to reduce benzoate and phenylbutyrate in the blood - ○ to elevate blood creatine kinase levels **What caused the ammonia levels to rise in the experimental group?** - ○ oxidative deamination of glutamate - ○ increased glycolytic activity - ○ increased gluconeogenic activity - ○ enzymatic degradation of α-ketoglutarate --- **Why did the absence of arginine lead to ammonia toxicity?** - ○ Arginine allosterically activates carbamoyl phosphate synthetase I. - ○ Arginine facilitates the metabolism of NH₄⁺ in the urea cycle. - ○ Arginine facilitates the metabolism of NH₃ in the urea cycle. **Is arginine an essential amino acid in cats? Why or why not?** - ○ Arginine is not an essential amino acid because ornithine in the diet effectively replaces it. - ○ Arginine is an essential amino acid because synthesis is not sufficient to meet the cats’ needs. - ○ Arginine is an essential amino acid because ornithine is not sufficient to replace it. **Why did the absence of arginine lead to ammonia toxicity?** - Arginine allosterically activates carbamoyl phosphate synthetase I. - Arginine facilitates the metabolism of \( \text{NH}_4^+ \) in the urea cycle. - Arginine facilitates the metabolism of \( \text{NH}_4^+ \) in the citric acid cycle. - Arginine transports \( \text{NH}_3 \) to the liver. **Is arginine an essential amino acid in cats? Why or why not?** - Arginine is not an essential amino acid because ornithine in the diet effectively replaces it. - Arginine is an essential amino acid because its synthesis is not sufficient to meet the cats’ needs. - Arginine is an essential amino acid because overnight fasting depletes the blood’s amino acid levels. - Arginine is not an essential amino acid because ornithine has the same metabolic function as arginine. **Why can ornithine substitute for arginine?** - Ornithine is the immediate precursor to arginine. - Ornithine is a synthetic version of arginine. - The citric acid cycle converts ornithine to arginine. - The urea cycle converts ornithine to arginine.