BIOSYNTHETIC-STUDIES ON THE XANTHONE - ANTIBIOTICS LYSOLIPIN-X AND LYSOLIPIN-I

Feeding experiments with C-13 and O-18-labeled precursors revealed tha t the molecular framework of the polycyclic xanthone antibiotics, the lysolipins X (1) and I (2), is derived from the polyketide pathway (12 malonate unites), the C-1 pool (methionine), molecular oxygen, and th e nitrogen pool. Surprisingly, and intact malonate moiety serves as th e three-carbon starter unit of the polyketide backbone, and 9 of the 1 2 oxygen atoms of 1 originate from molecular oxygen, including both of the xanthone oxygen atoms. The orientation of the malonate unit incor porated intact into lysolipin is unique and opposite from those in tet racycline and cycloheximide, i.e., the activated carbon of malonyl CoA is bound to the nitrogen of the lysolipin isoquinoline ring and the C O2-derived carbon serves as the starter of the polyketide chain. From the biogenetic origin of the oxygen atoms several unusual prearomatic deoxygenation steps early in the biosynthesis have to be postulated.