New milbemycins from Streptomyces hygroscopicus subsp. aureolacrimosus: fermantation, isolation and strucutre elucidation.

Twelve new milbemycins have been isolated and characterized from some strains derived from Streptomyces hygroscopicus subsp. aureolacrimosus SANK 60286 and SANK 60526. The metabolites 1 approximately 4 and 9 approximately 11 were produced by strain RM28D-688 SANK 60797 as minor products. The metabolites 5 approximaetly 8 were obtained from a broth of strain 57-338 SANK 61796. Strain MK-1391 SANK 62896 was used for the production of metabolite 12. The new metabolites, eight alpha-class and four beta-class compounds, have new structural features. For example, milbemycins alpha26 and alpha27, have the 26-hydroxy moiety, and other derivatives (milbemycins alpha20 approximately 23) have different side chains at the C-26 position from those of milbemycins alpha11 and alpha14. In addition, 5-hydroxylmilbemycin beta7 (beta12), involved in the major biosynthetic pathway of 25-methyl and 25-ethyl milbemycins, was discovered.

Bioconversion of milbemycin-related compounds: isolation and utilization of non-producer, strain RNBC-5-51.

A non-producing strain, the so-called strain RNBC-5-51 SANK 60198, was isolated during a screening program of strain improvement in the milbemycin production. Strain RNBC-5-51 indicated almost the same characteristics as those in the parent strain, that is, the abundant spore formation on agar media and the good growth in liquid media. But it does not produce any kind of milbemycins. In addition, strain RNBC-5-51 accumulated precursor-like compounds of milbemycin-polyketide, the production of which were inhibited by the addition of cerulenin. In the bioconversion experiments, strain RNBC-5-51 converted milbemycin beta6 and A4 to milbemycin alpha14, and milbemycin beta7 and A3 to milbemycin alpha11, respectively. This strain also converted milbemycin D and avermectin B1a, to 26-(3-methyl-2-butenoyloxy)milbemycin D and 26-(3-methyl-2-butenoyloxy)avermectin B1a, respectively. These results suggest that milbemycin alpha11 is biosynthesized through the same route as milbemycin alpha14, and the mutated step in strain RNBC-5-51 might be in the polyketide synthetic pathway of milbemycins. Strain RNBC-5-51 loses the ability for de novo synthesis of milbemycins, but it retains the ability to bioconvert the milbemycin skeleton. This strain might be useful for C-26 modification of milbemycin-related compounds.

Bioconversion of milbemycin-related compounds: biosynthetic pathway of milbemycins.

Streptomyces hygroscopicus subsp. aureolacrimosus SANK 60286 and SANK 60576 produce many kinds of milbemycins. Among them, milbemycin alpha11, alpha14, A3, and A4 have the most effective acaricidal activity. In this study, we investigated the terminal biosynthetic pathway to milbemycin alpha14 and A4 which accumulated as the final products in these strains. Using cerulenin, a specific inhibitor of fatty acid and polyketide biosynthesis, we conducted bioconversion experiments with cultures of several mutants, including milbemycin A4- and alpha14-producing strains. The bioconversions of milbemycin beta6 to milbemycin A4 and milbemycin A4 to milbemycin alpha14 could be identified. For the biosynthesis of milbemycin A4 from milbemycin beta6 in the milbemycin A4-high producing strain, there appeared to be two separate pathways exhibiting different sequences of furan ring formation and C-5 keto reduction steps.