Bioactive Agents from Natural Sources: Trends in Discovery and Application - Advances in Biochemical Engineering Biotechnology - page 127

ly those of microbial secondary metabolism.Of the many well-known advanta-
ges of the use of enzymes as catalysts,the high chemo-,regio- and stereoselec-
tivity,as well as mild reaction conditions are particularily advantageous when
used with natural products. Comments on the most wide spread methods of
biological derivatization which are based mainly on the use of microorganisms
are summarized below:
i) Microbial transformation makes use of enzyme catalyzed reactions with
living cells, typically exploiting a single chemical reaction, like oxidation,
reduction, hydrolysis, acylation, phosphorylation, glycosylation, methyla-
tion, amination, halogenation, isomerization or formation of N-oxides
[244-246].The methods of application,as well as the large variety of types
of microbial transformation reactions are subject of some excellent review
articles [242,245-256].There exist examples of highly complex molecules
where microbial oxygenation has been shown,e.g.hydroxylation reactions
ofthe macrolide antibiotics avermectin and rapamycin [257-259],hydroxy-
lation of the polyether grisorixin [260], and carbonyl reduction of mide-
camycin A 3 [261].In general,experiments of microbial transformation are
easy to handle.However,the prerequisite is a screening ofa large number of
organisms for a desired enzyme activity.
ii) In precursor-directed biosynthesis the derivatization of a secondary meta-
bolite is achieved by adding analogues of a biosynthetic precursor to the
cultivation of a producing organism [262]. Thus, organisms capable of
incorporating artificial precursors into their enzymatic processes provide
modified metabolites.Precursor-directed biosynthesis requires knowledge
about the biosynthesis of the original product,especially the basic building
blocks and major biosynthetic intermediates.The method has been success-
fully introduced to industrial antibiotic production of penicillins, amino-
glycoside antibiotics and avermectins,among others.Most examples ofpre-
cursor-directed biosynthesis, however, are of exploratory nature, and pro-
ducts are obtained in lab-scale amounts.
iii) Metabolic manipulation is based on the variation of cultivation parameters
of antibiotic producing microorganisms. Such manipulations of the bio-
synthetic machinery have resulted in the production of new metabolite
derivatives,presumably involving post-genetic alterations.The variations of
cultivation parameters are supposed to be drastic changes (”stress fermen-
tation”) rather than parameter variations as in optimization experiments of
cultivations. Such changes in cultivation parameters that might influence
metabolite patterns are temperature as in heat shock or low temperature
fermentation [263]),mechanical stress (e.g.changes in rheology or addition
of glass beads),light [264],pH-value,addition of ingredients to the culture
broth [265, 266], or increase of oxygen partial pressure up to 1,200 mbar
[267-272].
iv) Enzyme inhibition as a biological derivatization strategy works by blockage
of a particular biosynthetic pathway through administration of an enzyme
inhibitor to the growing culture. Generally, this method can yield three
different kinds of products: biosynthetic intermediates which are accumu-