Unambiguous Characterization of Commercial Natural (Dihydro)phenanthrene Compounds Is Vital in the Discovery of AMPK Activators

These days, easy access to commercially available (poly)phenolic compounds has expanded the scope of potential research beyond the field of chemistry, particularly in the area of their bioactivity. However, the quality of these compounds is often overlooked or not even considered. This issue is illustrated in this study through the example of (dihydro)phenanthrenes, a group of natural products present in yams, as AMP-activated protein kinase (AMPK) activators. A study conducted in our group on a series of compounds, fully characterized using a combination of chemical synthesis, NMR and MS techniques, provided evidence that the conclusions of a previous study were erroneous, likely due to the use of a misidentified commercial compound by its supplier. Furthermore, we demonstrated that additional representatives of the (dihydro)phenanthrene phytochemical classes were able to directly activate AMPK, avoiding the risk of misinterpretation of results based on analysis of a single compound alone.

Secondary Metabolites of Plants

Orchid phytochemicals that have been studied include alkaloids, bibenzyls, phenanthrenes, stilbenoids, phenols, flavonoids, anthocyanins and polysaccharides. Many of these compounds exhibit antioxidant, anti-inflammatory, antimicrobial, antihelminthic, anticoagulant, antidiabetic and lipid-lowering properties. Some compounds are cytotoxic, prevent angiogenesis and tumour spread and promote programmed cell death of cancer cells. Some compounds protect nerve cells against chemicals and oxygen deprivation (stroke), promote nerve cell regeneration, protect the skin from ultraviolet damage, protect the liver against poisons such as carbon tetrachloride, prevent calcium loss from bone and enhance foetal lung maturation. This chapter provides a short account of the characteristics of the various groups of compounds, including their properties.

9,10-Dihydrophenanthrenes as phytoalexins of Orchidaceae. Biosynthetic studies in vitro and in vivo proving the route from L-phenylalanine to dihydro-m-coumaric acid, dihydrostilbene and dihydrophenanthrenes

Hydroxy derivatives of 9,10-dihydrophenanthrenes, orchinol and hircinol, were isolated from bulbs of Orchidaceae which had been induced to accumulate phytoalexins. Incorporation of radioactive precursors, L-phenylalanine and various hydroxycinnamic acids, has been investigated by feeding experiments in vivo. m-Coumaric acid and dihydro-m-coumaric acid were found to be efficiently incorporated into the dihydrophenanthrene derivatives. Dihydro-m-coumaric acid was not only converted into the dihydrophenanthrenes but was also formed from L-phenylalanine in the same tissue; it was thus proved to be an intermediate. The role of dihydro-m-coumaric acid was substantiated by studies in vitro. An active stilbene synthase was detected in enzyme preparations from induced orchid bulbs and assayed with different CoA esters. The enzyme, characterized on the basis of its substrate specificity, selectively converted dihydro-m-coumaroyl-CoA plus malonyl-CoA into 3,3',5-trihydroxybibenzyl. The role of 3,3',5-trihydroxybibenzyl as physiological intermediate was further corroborated by investigations with intact plants. Both its formation from phenylpropanoids and its conversion into orchinol was demonstrated. The data provided evidence for a biosynthetic sequence originating from L-phenylalanine and leading to 9,10-dihydrophenanthrenes via m-coumaric acid, dihydro-m-coumaric acid, and 3,3',5-trihydroxybibenzyl.