The Journal of Antibiotics Biosynthesis of Antibiotic A23187 Incorporation of Precursors into A23187 Note

Streptomyces chartreusis NRRL 3881 produces the divalent cation ionophore antibiotic A231871). This antibiotic is primarily active against Grampositive bacteria and fungi and has been used extensively to investigate the role of divalent cations in regulating cellular physiological processes2,3,4). The chemical structure of A231875.6) indicates that it is a member of a new class of ionophorous antibiotics, the pyrrole polyethers.7) The only other antibiotic in this class is the recently discovered X-14547A8). In this report, the incorporation and labeling pattern of 14Cand 13C-labeled precursors into A23187 is described. Antibiotic A23187 (1a) is a mono-carboxylic acid consisting of three structural units; an aketopyrrole, a substituted benzoxazole and a spiroketal rings'. The a-ketopyrrole moiety could arise from proline or acetate while the spiroketal ring system could be formed by propionate and acetate9) or by condensation of acetate units with C-methyl groups arising through transmethylation from a C,-donor10). The aromatic ring of the benzoxazole moiety could be derived from 3-hydroxyanthranilic acid via tryptophanll) or from glucose metabolism via a shikimic type intermediate"). These possibilities were tested by determining the incorporation of 14C-labeled precursors into the antibiotic (Table 1). Radioisotope labeled substrate (10 p.Ci to 50 1 Ci/50 ml culture broth) were added to individual cultures of S. chartreusis three days after inoculation. Cultures were harvested twentyfour hours later and A23187 was isolated, converted to its acid form, methylated with diazomethane, and purified by chromatography as described elsewhere". The results (Table 1) indicated that methionine, propionate, acetate, glycerol, glucose, proline and ornithine were all efficiently incorporated into A23187 methylester (1b). Information on the labeling pattern of these precursors was obtained by chemical degradation of the specifically radioisotope labeled antibiotic (Table 2). KUHN-ROTH oxidation" of the antibiotic made possible the isolation of carbons 19' and 19, 17' and 17, 15' and 15, and 11' and 11 in the form of sodium acetate. SCHMIDT degradation" of the sodium acetate derived from the K-R oxidation yields monomethyl amine and CO2, from C-2 and C-1 of the acetate, respectively. The results (Table 2) indicate that the C-3 and C-2 of propionate are the source of the methyl and adjacent methylenes of the spiroketal ring of A23187. The specific activity of the sodium acetate derived from A23187 labeled from propionate [3-14C] and [2-14C] is one quarter that of the labeled antibiotic. Table 1. Incorporation of 14C-labeled substrates into A23187 methylester.