2. In a study conducted some years ago, cats were fasted overnight then given a

Question

2. In a study conducted some years ago, cats were fasted overnight then given a single meal complete in all amino acids except arginine. Within 2 hours, blood ammonia levels increased from a normal level of 18 g/L to 140 g/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 arginine was replaced by ornithine (a) What caused the ammonia levels to rise in the experimental group? Why did the absence of arginine lead to ammonia toxicity? Is arginine an essential amino acid in cats? Why or why not? (b) Why can ornithine be substituted for arginine?

Explanation / Answer

a) Here the experimental group was provided with a diet that lacked Arginine amino acid. In cats, arginine is an essential amino acid. The term essential amino acid means, those amino acids that are to be supplied through diet as it cannot be synthesised in the body. In cats arginine acts as an essential amino acid because cats lack the enzyme pyrolline 5- carboxylate synthase. This enzyme is required to synthesize arginine precursor. Arginine amino acid is required for the elimination of urea from our body. It reduces the toxicicity of nitrogen compounds which are synthesised by the liver.

Here, in this case, as the aminoacid arginine was lacking, the urea from blood was not able to remove and this caused the increase in blood ammonia levels.

This can in turn result in liver dysfunction and this accumulation of nitrogenous compounds finally resulted in ammonia toxicity.

b) The amino acid arginine belongs to alpha ketoglutarate family. It can be synthesized from glutamate. Synthesis of ornithine from glutamate is a step in arginine biosynthesis. That is, ornithine acts as an intermediate in arginine biosynthetic pathway. Therefore, ornithine can be substituted for arginine.

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