STUDIES ON THE CELL-FREE SYNTHESIS OF ECHINOMYCIN AND AN ECHINOMYCIN ANALOGUEt

Echinomycin producing Streptomyces cultures (X-53 or X-63) grown in the presence of quinazol-4-one-3-acetic acid yield an analogue in which one of the two quinoxaline-2-carboxyl residues is replaced by a quinazol-4-one-3acetyl moiety. This analogue has been designated quinazomycin. Cell-free extracts of culture X-63 .synthesize echinomycin in the presence of precursors, ATP, Mg2 + and mercaptoethanol. Replacement of quinoxaline2-carboxylic acid in this solution with quinazol-4-one-3-acetic acid yields an echinomycin analogue with two quinazol-4-one-3-acetyl residues, designated biquinazomycin. The studies reported owe their origin to a hypothesis that the group of antibiotics having heterocyclic or other moieties attached to a peptide sequence, might owe their origin to detoxication of the heterocyclic or other moiety by peptidation by the antibiotic producing organism. If this is so, introduction of analogues of the heterocycle or other non-peptide portion of the antibiotic into the antibiotic synthesising system should lead to the formation of analogues of the antibiotic. As echinomycin producing cultures were available to us (X-53' and X-632) studies with these cultures were undertaken. Echinomycin is a quinoxaline antibiotic isolated from several Streptoinyces cultures. Although it possesses antitumour, antiviral and antibacterial activities3' , its low therapeutic index has precluded any utility. The structure (I, R1 R2 II) of this antibiotic was established by Prelog and his associates5. This symmetrical dimeric molecule has an unusual cyclic octapeptide ring made up of two residues each of Ser, Ala, N-methylhomocysteine and N-methylvaline. The homocysteine residues are joined to form a dithiapyran ring and the octapeptide to two quinoxaline-2-carboxylic acid (II) residues through amide bonds. A further reason for attempting to obtain analogues of echinomycin was the further report6 that an echinomycin producing culture produced analogues in the presence of quinaldinic acid. t Communication No. 1606 from the Central Drug Research Institute. These studies have been reported. For further experimental details see references 7 and 8.