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Analysis of the reaction between 2'-deoxycytidine and 4-oxo-2-nonenal by LC/MS revealed the presence of three major products (adducts A(1), A(2), and B; [M + H](+) = 364). Adducts A(1) and A(2) were isomeric, and each dehydrated to form adduct B. The structure of adduct B was shown by LC/MS and NMR spectroscopy to be an etheno-2'-deoxycytidine adduct 1' '-[1-(2'-deoxy-beta-d-erythro-pentofuranosyl)-1H-imidazo[2,1-c]pyrimidin-2-oxo-4-yl]heptane-2' '-one. A time course experiment performed at 65 degrees C (pH 5-8) showed that the transformation of both A(1) and A(2) was pH-dependent. In acidic conditions, adducts A(1) and A(2) dehydrated primarily to adduct B. In contrast, in basic conditions, adducts A(1) and A(2) hydrolyzed primarily to dCyd. The data are consistent with adducts A(1) and A(2) being substituted ethano adducts that dehydrate to adduct B, a substituted 3,N(4)-etheno-2'-deoxycytidine adduct.

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Synthesis and resolution of the antibiotic phosphonomycin are described. The structure is (-)(IR, 2S)-1,2-epoxypropylphosphonic acid.

A second open reading frame, fkbD, was found upstream of fkbM in all three aforementioned species and was predicted to encode a protein of 388 residues that showed a strong resemblance to cytochrome P-450 hydroxylases. Disruption of fkbD had a polar effect on the synthesis of the downstream fkbM gene product and resulted in the formation of 9-deoxo-31-O-demethyl-FK506. This established the product of fkbD as the cytochrome P-450 9-deoxo-FK506 hydroxylase, which is responsible for hydroxylation at position C-9 of the FK506 and FK520 macrolactone ring.The polyketide, immunosuppressant compound FK506 ( Fig. 1) (13) is a 23-membered macrolide with potent antifungal activity produced by several Streptomyces species. FK506 is approximately 100-fold more potent than the structurally unrelated immunosuppressive compound cyclosporin A. Both drugs are important therapeutic agents for the prevention of graft rejection following organ and bone marrow transplantations and for the treatment of autoimmune diseases (22). FK520 (also known as immunomycin and ascomycin) is another immunosuppressive compound similar to FK506 (Fig. 1) in which the allyl group is replaced by an ethyl group at position C-21 of the macrolactone ring (9). Both the antifungal and the immunosuppressive activities of FK520 are approximately one-half of those exhibited by FK506 (9).Through precursor incorporation experiments, Byrne et al. (3) demonstrated that the polyketide portion of FK506 and FK520 is derived, for the most part, from acetate and propionate. Those authors also established the origin of the pipecolate and the cyclohexyl rings to be lysine and shikimic acid, respectively, and demonstrated that the source of the methyl portion of the methoxyl groups at C-13, C-15, and C-31 of FK520 (Fig. 1) is L-methionine.The enzymology of FK506 biosynthesis has also been explored to some extent. The pipecolate-activating enzyme which presumably incorporates pipecolate into the completed polyketide chain has been characterized previously (19). Both 31-O-demethyl-FK520 methyltransferase and 31-O-demethyl-FK506 methyltransferase (FKMT) have been isolated from the producing strains (3, 27). These two enzymes can use each other's substrate interchangeably and methylate the C-31 OH and not the C-13 or C-15 OH group (27).Here, we report the isolation and molecular characterization of two genes involved in the biosynthesis of FK506. One gene, fkbM, encodes FKMT, and the other, fkbD, encodes a cytochrome P-450 9-deoxo-FK506 hydroxylase that catalyzes hydroxylation at C-9. MATERIALS AND METHODSStandard recombinant DNA techniques were performed as described by Sambrook et al. (24).Probe design. N-terminal amino acid sequencing of FKMT from Streptomyces sp. strain MA6858 (27) gave a 39-mer with the sequence SDVVETLRLPNGA TVAHVNAGEAQFLYREIFTDRCYLRH. This peptide sequence was then used to design two nonoverlapping degenerate oligonucleotide probes, P1 and P2, in which inosine was incorporated at the third position of highly degenerate codons (2). P1 corresponded to ...

1. Ivermectin was extensively metabolized by human liver microsomes to at least 10 metabolites. The structure of many of them (mostly hydroxylated and demethylated) was determined by 1H-NMR and LC/MS. 2. To determine which human cytochrome P450 isoform(s) is responsible for the metabolism of ivermectin, chemical inhibitors including sulphaphenazole, quinidine, furafylline, troleandomycin (TAO) and diethyldithiocarbamate (DDC) were used to evaluate their effect on ivermectin metabolism. TAO, a specific inhibitor of cytochrome P4503A4, was the most potent inhibitor, inhibiting the total metabolism as well as formation of each metabolite. Metabolism was also inhibited by an anti-human cytochrome 3A4 antibody by 90%. 3. When ivermectin was incubated with microsomes from cells expressing CYP1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1 or 3A4 at 4 mg/ml protein concentrations, metabolic activity was only detected with the microsomes containing CYP3A4. The metabolic profile from cDNA-expressed CYP3A4 microsomes was qualitatively similar to that from human liver microsomes. 4. Thus, cytochrome P4503A4 is the predominant isoform responsible for the metabolism of ivermectin by human liver microsomes.

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