Utilization of l-threonine by a species of Arthrobacter. A novel catabolic role for ‘aminoacetone synthase’

Abstract

1. A species of Arthrobacter (designated Arthrobacter 9759) was isolated from soil by its ability to grow aerobically on l-threonine as sole source of carbon atoms, nitrogen atoms and energy; the organism also grew well on other sources of carbon atoms including glycine, but no growth was obtainable on aminoacetone or dl-1-aminopropan-2-ol. 2. During growth on threonine, 14C from l-[U−14C]threonine was rapidly incorporated into glycine and citrate, and thereafter into serine, alanine, aspartate and glutamate. 3. With extracts of threonine-grown cells supplied with l-[U−14C]threonine, evidence was obtained of the NAD and CoA-dependent catabolism of l-threonine to produce acetyl-CoA plus glycine. Short-term incorporation studies in which [2−14C]acetate and [2−14C]glycine were supplied (a) to cultures growing on threonine, and (b) to extracts of threonine-grown cells, showed that the acetyl-CoA was metabolized via the tricarboxylic acid cycle and glyoxylate cycle whereas the glycine was converted into pyruvate via the folate-dependent ‘serine pathway’. 4. The threonine-grown organism contained ‘biosynthetic’ threonine dehydratase and a potent NAD-linked l-threonine dehydrogenase but possessed no l-threonine aldolase activity. 5. Evidence was obtained that the acetyl-CoA and glycine produced from l-threonine had their immediate origin in the α-amino-β-oxobutyrate formed by the threonine dehydrogenase; the CoA-dependent cleavage of this compound was catalysed by an α-amino-β-oxobutyrate CoA-ligase, which was identified with ‘aminoacetone synthase’. A continuous spectrophotometric assay of this enzyme was developed, and it was found to be inducibly synthesized only during growth on threonine and not during growth on acetate plus glycine. 6. By using a reconstituted mixture of separately purified l-threonine dehydrogenase and α-amino-β-oxobutyrate CoA-ligase (i.e. ‘aminoacetone synthase’), l-[U−14C]threonine was broken down to [14C]glycine plus [14C]acetyl-CoA (trapped as [14C]citrate). 7. There was no evidence of aminoacetone metabolism by Arthrobacter 9759 even though a small amount of this amino ketone appeared in the culture medium during growth on threonine.