Enzymatic degradation of bacterial poly(3-hydroxybutyrate) by a depolymerase from Pseudomonas lemoignei

Extracellular poly(3-hydroxybutyrate)(PHB)-depolymerases A and B were purified from the culture medium of succinate-grown cells of Pseudomonas lemoignei by ammonium sulfate precipitation and chromatography on CM Sepharose CL-6B. The relative molecular weights of the sodium dodecyl sulfate (SDS)-treated enzymes were estimated to be 67 000 for both depolymerases. PHB-depolymerase A was homogeneous, as revealed by SDS-PAGE, and was used for degradation experiments on compression molded PHB films, exhibiting a spherulitic morphology. The extent of enzymatic degradation was found to increase with increasing depolymerase concentration (C-E) and reach a plateau at C-E greater than or equal to 3 mu g/mL. Slight morphological changes were induced on PHB films by different thermal treatments, in order to study their effect on degradation rate at a constant overall degree of crystallinity. It was found that the enzymatic degradation rate of PHB decreases as a consequence of an increase of the average size of the crystals, i.e. of crystalline phase perfection. The surface morphology of compression-molded PHB films after enzymatic degradation, investigated by scanning electron microscopy, is discussed in terms of preferential degradation of the amorphous phase.