Poly (L‐lactide)‐Degrading Enzyme Produced by Amycolatopsis sp.

Poly (L ‐lactide) (PLLA)‐degrading enzyme was produced in a liquid culture of Amycolatopsis sp. (strain 41). In comparison with polyester substrates, silk powder from silkworm cocoons was the most effective in inducing enzyme production within 5 d. Application to DEAE and Superdex 75 columns resulted in a major protein with molecular weight estimated to be 42 kDa from size exclusion chromatography or 40 kDa from SDS‐PAGE analysis. Optimum pH and temperature are 6.0 and 37–45°C, respectively. Besides PLLA, the enzyme degrades casein, silk powder and Suc‐(Ala)₃‐pNA at an even lower level than Proteinase‐K, but not Suc‐(Gly)₃‐pNA, poly (ε‐caprolactone) and poly (β‐hydroxybutyrate).     

Poly(L‐lactide)‐Degrading Activity in Various Actinomycetes

The poly(L ‐lactide) (PLA)‐degrading ability of actinomycetes obtained from culture collections was examined by the formation of clear zones on PLA‐emulsified agar plates. Using 41 genera (105 strains) of actinomycetes with phylogenetic affiliations based on 16S rRNA sequences, PLA degraders were found to be limited to members of the family Pseudonocardiaceae and related genera. They included Amycolatopsis, Saccharothrix, Lentzea, Kibdelosporangium, and Streptoalloteichus. A large number of PLA degraders were widely distributed within the genus Saccharothrix. Most strains forming clear zones on PLA‐emulsified agar plates also formed clear zones on silk fibroin agar plates. Saccharothrix species showed an ability to degrade PLA films and assimilate degradation products in liquid cultures. No significant change of the molecular weight and polydispersity (M _w/M _n) of the remaining film fragments was confirmed. After cultivation for two weeks, many irregular holes/pits on the surface of the film due to the colonization of microorganisms were observed by scanning electron microscopy.

Scanning electron micrograph of the surface of PLA film: A. orientalis subsp. orientalis IFO 12362 after 14 d.