Some similarities to:
Motherwort but the leaves of Motherwort are very different, having three lobes.
Not to be semantically confused with : Hound's-tongue [a plant with similar name]
Not to be confused with the closely related species: White Horehound (Murrubium vulgare) which is in a differing genus (within the same family), thyme scented and is so smothered in white hairs as to appear grey-green.
Many similarities to Hedge Woundwort but that has deeper purple flowers with white markings on the lower lip, which is also not lobed as conspicuously as is Black Horehound. Also, Hedge Woundwort has no sterile sepal tubes; all will have flowers in them. Black Horehound has many more flowers in the whorl. Both smell pretty dire.
Black Horehound is renown for its highly disagreeable pungent smell when crushed, but some folk actually like this aroma. So far, your author has been unsuccessful at finding the chemical compound(s) responsible for this foetid smell, but several candidates follow.
DITERPENOID LACTONES of the LABDANE TYPE
Black Horehound contains half a dozen Diterpenoid lactones of the Labdane type, compare Larixol. Most have an additional Furan ring (top right), or two in the case of
Preleosibirin. All are loosely based on
Marrubiin is also found in White Horehound (Marrubium vulgare), a related species. It is this species which used to be used in cough candy as an expectorant, although nowadays perhaps less toxic alternatives are used, i.e. purer ingredients rather than incorporating the essential oil from the whole plant(?).
One reference quotes Black Horehound as not possessing
Ballonigrin (despite the name) and instead says it is present in several other Ballota species instead, but not Ballota nigra.
Preleosibirin is a Prefuranoid with, unlike the above, two acetyl groups (lower left) and two five-membered rings (upper right). It can easily be envisaged that the carbon chain linking the six-membered rings to the five-membered ring of the other five comes about as a result of the breaking of the oxygen bridge bond on the first five-membered ring (
TetraHydroFuran, but your Author cannot be sure.
Black Horehound also contains the flavonoid
Luteolin-7-O-Lactate and its glycopyranoside as well as a poly-methoxylated flavone called
Tangeritin, which is present in the peel of
Tangerines and other citrus fruits. Tangeritin strengthens the cell walls and also has a cholesterol-lowering effect in the human body, and may also be useful against Parkinson's Disease. It will selectively induce apoptosis in Leukaemic cancer cells whilst ignoring normal cells and has great potential for use against cancer, although it counteracts the laboratory-synthesized anti-cancer drug Tamoxifen which is already utilised medicinally to suppress the activity of natural killer cells.
The flavonoids showed some anti-fungal activity against Candida species.
PHENYL PROPANOIDS and their GLYCOSIDES
These glycosides are all based upon the non-glycoside (+)-(E)-Caffeoyl-(L)-Malic Acid, which is also present in Black Horehound.
Black Horehound also contains the following PhenylPropanoid Glycosides within the flowers: Verbascoside,
Angoroside A. These compounds are responsible for the neurosedative activity of Back Horehound, which prolongs sleep. They are able to bind to dopaminergic, benzodiazepine and morphinic receptors. The two glycosidic units present in Verbascoside and all the other Phenylpropanoid Glycosides shown here are glucopyranose and mannopyranose.
Some of these compounds demonstrated anti-fungal activity, with Arenarioside being effective against methicillin-resistant staphylococcus (MRSA). The first one, Verbascoside, has but two glycosidic units, the next five have three glycosidic units, whilst the last, Balletetroside, has four.
Pinene and Humulene have also been found in the volatile oil from Black Horehound.