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COO" Oxalate ketobutyrate by the action of threonine dehydrogenase. This is a relatively minor pathway in humans, accounting for 10% to 30% of threonine catabolism, but is more important in some other mammals. The major pathway in humans leads to succinyl-CoA and is described later.

In the laboratory, serine hydroxymethyltransferase will catalyze the conversion of threonine to glycine and acetaldehyde in one step, but this is not a significant pathway for threonine degradation in mammals.

TThe primary function of D-amino acid oxidase, present at high levels in the kidney, is thought to be the detoxification of ingested D-amino acids derived from bacterial cell walls and from cooked foodstuffs (heat causes some spontaneous racemization of the lamino acids in proteins). Oxalate, whether obtained in foods or produced enzymatically in the kidneys, has medical significance. Crystals of calcium oxalate account for up to 75% of all kidney stones. ■

There are two significant pathways for threonine degradation. One pathway leads to pyruvate via glycine (Fig. 18-19). The conversion to glycine occurs in two steps, with threonine first converted to 2-amino-3-

Seven Amino Acids Are Degraded to Acetyl-CoA

Portions of the carbon skeletons of seven amino acids— tryptophan, lysine, phenylalanine, tyrosine, leucine, isoleucine, and threonine—yield acetyl-CoA and/or acetoacetyl-CoA, the latter being converted to acetyl-CoA (Fig. 18-21). Some of the final steps in the degrada-tive pathways for leucine, lysine, and tryptophan resemble steps in the oxidation of fatty acids. Threonine (not shown in Fig. 18-21) yields some acetyl-CoA via the minor pathway illustrated in Figure 18-19.

The degradative pathways of two of these seven amino acids deserve special mention. Tryptophan breakdown is the most complex of all the pathways of amino

Tryptophan

51 Tips for Dealing with Kidney Stones

51 Tips for Dealing with Kidney Stones

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