[MELLE(4)]CYCLOSPORIN, A NOVEL NATURAL CYCLOSPORINE WITH ANTI-HIV ACTIVITY - STRUCTURAL ELUCIDATION, BIOSYNTHESIS

From fermentations of Tolypocladium niveum supplemented with D-threoni ne, a novel natural cyclosporin, [Melle(4)]cyclosporin, was isolated. Its structural elucidation is based on amino acid analysis and spectro scopic data; the amino acid sequence was deduced from two-dimensional NMR investigations applied to the iso-derivative of [Melle(4)]cyclospo rin which, in contrast to the natural product, is present as one homog enous conformation in solution. We show that one of the four N-methyl- L-leucine units of cyclosporin A, namely that in position 4, is replac ed by N-methyl-L-isoleucine. The putative mechanism by which D-threoni ne induces in vivo biosynthesis of [Melle(4)]cyclosporin is discussed. In vitro biosynthesis of [Melle(4)]cyclosporin was achieved using the previously described enzymatic system [Lawen and Traber (1993) J Biol Chem 268: 20452-20465], thereby demonstrating the high affinity of cy closporin synthetase for isoleucine in position 4. In a long series of cyclosporins obtained by in vitro and in vivo biosynthesis, [Melle(4) ]cyclosporin represents the first example that is devoid of immunosupp ressive efficacy while retaining strong binding to cyclophilin. It exe rts potent in vitro anti-HIV-l activity.