Secondary metabolites definition

Growth and diet are subject to seasonal changes. The presence (or absence) and relative concentrations of secondary metabolites in the extracts of organisms are factors which chemists should document along with observations made during collection. Often, in preliminary experiments guided by bioassay, mixtures, rather than individual compounds, are tested. Association of a certain activity with structurally related compounds may serve as a lead for further tests, but should not be considered definitive. 

The strict definition of a fungal secondary metabolite is a compound that is produced as a result of a nutrient limitation. 

The concept of primary and secondary metabolites is outlined and it is concluded that an exact definition is not especially useful. The coexistence of metabolites and the cellular machinery for their transformation is outlined by the concept of com-partmentation (Appendix 1). Tertiary metabolites and xenochemicals are defined for the sake of completeness (Section 2.1) and the current theories of the function of secondary metabolites are briefly discussed (Section 2.2). 

An exact definition of primary and secondary metabolites is not very useful since it is deemed to be artificial. Although primary metabolism is to some extent similar in all organisms there are variations, major (photosynthetic/non-photosynthetic - aerobic/anaerobic organisms) as well as minor (bromotyrosine in sponge skeletons, ceramide aminoethylphosphonate in sea anemones (10)). This means that a secondary metabolite for one organism may assume the role as a primary one for another. From a purely formal point of view the distinction is meaningless and best abandoned. For classification purposes it still serves to delineate a large body of biochemicals in a convenient compilation. 

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