944

PART 4 Regulation and Maintenance

acid, and acetone are called ketone (k ē ′t ō n) bodies; they are releasedinto the blood, where they travel to other tissues, especially skeletalmuscle. In these tissues, the ketone bodies are converted back intoacetyl-CoA, which enters the citric acid cycle to produce ATP.The presence of small amounts of ketone bodies in the bloodis normal and beneficial, but excessive production of ketone bod-ies is called ketosis (k ē -t ō ′sis). Because ketone bodies are acidic,if the increased number of ketone bodies exceeds the capacity ofthe body’s buffering systems, acidosis, a decrease in blood pH,can occur (see chapter 27). Because ketone bodies are derivedfrom the breakdown of fatty acids, conditions that increase lipidmetabolism can speed the rate of ketone body formation, thusincreasing the possibility of ketosis. Examples of conditions thatincrease lipid metabolism are starvation (see Clinical Impact,“Starvation”), diets consisting mainly of proteins and lipids withfew carbohydrates, and untreated diabetes mellitus. Ketone bodiesare excreted by the kidneys and diffuse into the alveoli of thelungs. Because ketone bodies are excreted by the kidneys andlungs, the characteristics of untreated diabetes mellitus includeketone bodies in the urine and “acetone breath.”

ASSESS YOUR PROGRESS

38. What is beta-oxidation? Explain how it results in ATPproduction.

39. What are ketone bodies, how are they produced, and forwhat are they used? What occurs when there is an excess?

25.5 Protein Metabolism

LEARNING OUTCOME

After reading this section, you should be able to

A. Describe the basic steps involved in using proteins asan energy source.

Recall that proteins are polymers of amino acids. During diges-tion and absorption in the small intestine, proteins are brokendown to individual amino acids. Once absorbed into the body,amino acids are quickly taken up by cells, especially in the liver.Amino acids are used primarily to synthesize needed proteins(see chapter 3), and only secondarily as a source of energy(figure 25.11). Unlike glycogen and triglycerides, amino acidsare not stored in the body.As stated earlier in the chapter, the body can manufacturesome amino acids. The synthesis of nonessential amino acids usu-ally begins with keto acids. A keto acid can be converted into anamino acid by replacing its oxygen atom with an amine group(figure 25.12). Usually, this conversion is accomplished by trans-ferring an amine group from an amino acid to the keto acid, areaction called transamination (trans-am′i-n ā ′sh ŭ n). For exam-ple, α-ketoglutaric acid (a keto acid) reacts with an amino acid toform glutamic acid (an amino acid; figure 25.13 a ). Most aminoacids can undergo transamination to produce glutamic acid. The