acetyl CoA Figure 6 Metabolism of lysine.

Methionine and cysteine The conversion of methionine to cysteine via the so-called transsulfura-tion pathway has already been mentioned (Figure 5). This pathway appears to act mainly as a biosyn-thetic pathway for the synthesis of cysteine. There is an alternative pathway for methionine catabolism that involves transamination to methyl thio-a-oxo-butyrate and then to methyl thiopropionate.

Cysteine can be transaminated to thiopyruvate, which then undergoes desulfuration to pyruvate and hydrogen sulfide (Figure 5). Cysteine can also be oxidized to cysteine sulfinic acid, which can then be decarboxylated to hypotaurine, and this is then oxidized to taurine. High concentrations of taurine are found within most cells of the body, although its role is far from clear. In the liver the main fate of taurine is the production of taurocholic acid, which acts as an emulsifier in the bile. Another key role for cysteine is in the synthesis of the tripeptide glu-tathione, which is an important intracellular antioxidant.

Leucine, isoleucine, and valine The branched-chain amino acids are unusual in that the first step in their metabolism occurs in muscle rather than liver. This step is transamination, producing a-oxoisocaproic acid, a-oxo-^-methyl valeric acid, and a-oxoisova-leric acid. These ketoacids are then transported to the liver for decarboxylation and dehydrogenation. Subsequent catabolism yields acetyl CoA and acetoacetate in the case of leucine, acetyl CoA and propionyl CoA from isoleucine, and succinyl CoA from valine (Figure 7).

Histidine The first step in histidine metabolism is deamination to urocanic acid. Subsequent metabolism of this compound can follow several different pathways, but the major pathway is the one that involves formiminoglutamic acid (FIGLU), which is demethylated by a terahydrofolic acid-dependent leucine

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