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1. l-αγ-Diaminobutyric acid is metabolized in Xanthomonas sp. to aspartic β-semialdehyde, aspartic acid and oxaloacetic acid. 2. Aspartic β-semialdehyde is formed from diaminobutyric acid by a pyruvate-dependent γ-transamination. 3. The transaminase has a pH optimum of 9 and exhibits a high degree of substrate specificity, as analogues of diaminobutyric acid and pyruvate are inert in the system. The transaminase is inhibited by carbonyl-binding agents such as hydroxylamine. 4. Aspartic acid is formed from aspartic β-semialdehyde by an NAD+-dependent dehydrogenation. 5. The dehydrogenase has a pH optimum of 8·5 and is a thiol enzyme. It is specific for aspartic β-semialdehyde but analogues of NAD+ such as 3-acetylpyridine–adenine dinucleotide and deamino-NAD are partly active in the system. 6. The significance of these reactions is discussed in relation to diaminobutyric acid metabolism in plants and mammalian systems.

1. The activities of the enzymes histidase, urocanase and histidine-pyruvate transaminase were studied in rats under conditions of protein malnutrition. Urocanase and histidase activities in liver were markedly lowered in experimental protein malnutrition, but the activity of histidine-pyruvate transaminase was unaffected. There is a metabolic control in vivo of the enzymes involved in the catabolism of histidine. 2. Significant changes in the urinary excretion of histidine, composition of liver and serum were apparent in the protein-malnourished rat. 3. The changes in the activities of the enzymes and other parameters were of a reversible nature and dependent on the nature of the dietary protein. 4. The significance of these findings is discussed in relation to abnormal histidine metabolism in kwashiorkor.