Catabolism of galacturonic and glucuronic acids by Erwinia carotovora.

Our interest in the microbial breakdown of uranic acids stems from this group’s earlier studies (1,2) on the catabolism of pectin by soft-rot bacteria of the genus Erwinia. In those investigations it had been shown that Erwinia carotovora, when previously grown on pectin, possesses enzymes which can convert pectin to galacturonic acid, and that the galacturonate is further metabolized by galacturonate-grown cells to a mixture of end products similar to that formed by Aerobacter. Early experiments suggested that the first detectable step in the catabolism of n-galacturonate and n-glucuronate by cell-free extracts of E. carotovora and Aerobacter cloacae was a reduction of the aldehydic group with either DPNH or TPNH to the corresponding n-hexonic acid (3). We later found (4, 5) that extracts of Erwinia contain an enzyme which isomerizes galacturonic acid to 5-keto-n-galactonic acid; and glucuronic acid to 5-keto-n-gulonic acid.’ As indicated in a preliminary report, the isomerization is followed by the reduction of the keturonic acids to n-mannonic acid and n-altronic acid, respectively (6). The keturonic acid reductase, which catalyzes the reductive step, acts either with DPNH or TPNH. The n-hexonates are further metabolized to triosephosphate and pyruvic acid. The present paper summarizes our evidence for the identification of the intermediates and for the nature of the individual steps involved in the metabolic sequence. It has recently been found, independently of our own investigations, that extracts of uranic acid-adapted cells of Escherichia coli also catalyze the breakdown of uranic acids by this pathway (798).