Terpenes

 
There are a number of biological compounds, produced by plants and based on simple organic molecules of the aliphatic and aromatic series, that give rise to characteristic odours, taste, attractiveness (or otherwise) to insects and so on. We call these compounds terpenes and the potential number of them is probably almost infinite; certainly there are more of them than of any other plant or animal product.

For those interested in their chemistry they are based mainly on a simple carbon-5 isoprenoid unit. Monoterpenes have two ‘units’; there may be multiple chains of ‘units’ (see Figure). 

Derivation of phytol, found in green plants

 

Terpenes are characteristically plant products and are well-known as components of many essential oils. Camphor, rubber and carotene are terpenes. But monoterpenes have also been found in animals. Cholesterol is probably the best-known terpene. Citronellol has been isolated from the scent glands of alligators. Some insects produce monoterpenes as part of their defence secretions. Some of these compounds are endocrine precursors in animals. A lot of fascinating biochemistry lies before us and research is continuing.

TERPENES AND ALLERGIES
Resins and special oils in plants, especially coniferous trees, are terpenous. These can be severe allergens to certain individuals. The production of leaf terpene is greatest up to and just before the time of flowering. It then drops off rapidly. Here, then, is another seasonal allergen (see environmental allergies). Conifers, of course, are with us all year round so, even when growth is at a minimum, there is some exposure. It is believed that the typical blue haze over pine-covered mountains is due to the atmospheric terpenes they secrete.

It is worth reminding the reader that petrochemical products, which can be so troublesome to the allergically sensitive individual, all come ultimately from coniferous plants laid down some 200 million years ago. So there is little wonder that a reaction to ‘pine’ is quite common.

DESENSITIZATION
Some doctors report successful results neutralizing patients to terpenes using the Miller serial dilution technique. Flowers, leaves and other vegetable products are gathered and soaked with saline solution or Coca’s solution to extract the active chemicals. The resulting supernatant (liquid overlying the materials deposited by settling, precipitation or centrifuge) is filtered, diluted and injected for testing in the usual way (see Miller’s method). It can also be used sublingually (under the tongue).

‘Pine terpene’, a mixture of conifers, is important because it is so common an allergen. ‘Grass’ terpene is also significant. Most allergy doctors are aware that patients often react to grass that has been cut, without pollens being present. Dr Harry Morrow Brown finally proved this scientifically to everyone’s satisfaction. Therefore, replying only on pollen extract testing may miss the true cause of the patient’s reaction.

Terpene solutions to be used in testing can be made from all growing plants. A mixture of tree leaves is called ‘tree terpene’. Weeds can be made up separately or put together as a group. Flowering plants are used in their entirety; that is, leaves, petals, stamen and pistil. In fact, everything that grows in an area can be a ‘terpene’ solution for that locality. English doctors are now having to use ‘rape terpene’, as rape is a relatively new but powerful irritant in the UK.