Source, Etc

The official strophanthus seeds are obtained from Strophanthus Kombe, Oliver (N.O. Apocynaceoe), a climbing plant of considerable size, indigenous to eastern tropical Africa, near the Nyanza and Tanganyika lakes, the Shire river, etc. An extract prepared from them (and possibly from the seeds of other species of Strophanthus) is used in Africa as an arrow poison. Specimens of this extract were sent to England in 1861-64 and recognised by Sharpey (1862) as a cardiac poison. The seeds were examined by Fraser (1885), who isolated the active principle strophanthin, and subsequently recommended the seeds as a substitute for foxglove leaves.

The fruit of the plant consists of two follicles about 30 cm. in length and 2.5 cm. in breadth, slightly narrowed at the base and tapering at the apex. Each follicle contains, closely packed together, a large number of seeds provided with long awns. The fruits are collected when ripe, and are sometimes exported after having been freed from their epicarp and fleshy mesocarp. More commonly the seeds, separated from the fruits and deprived of their awns, are sent into commerce, chiefly from Somba, Quilimane, Inhambane, Chinde, and other East African ports.

Description

These very beautiful seeds are remarkable for the long plumose awn of white silky hairs that is attached to them. The integuments of the seed are prolonged at the apex to a slender brittle extension, which is terminated by a handsome feathery tuft of hairs about 5 cm. long and the same in breadth, the entire seed measuring about 10 cm. in length; these awns are commonly removed before the seeds are exported. The seeds as seen in commerce measure from 12 to 20 mm. in length and about 4 mm. in breadth. They are of an elongated oval shape, acuminate towards the apex, and narrowed towards the base, which is obtuse. They are flattened and provided on one side with a ridge running from the centre to the apex of the seed, and terminating in a broken point left by the removal of the awn. In colour they vary from greyish green to fawn; they are covered with satiny appressed hairs that impart to them a conspicuous sheen.

The seeds break easily; the transverse section exhibits a white oily kernel consisting of two straight cotyledons surrounded by a narrow endosperm. Moistened with 80 per cent, sulphuric acid the section assumes, at least in the endosperm but sometimes in the cotyledons also, a deep emerald green colour. The same reaction is afforded by the kernels from which the seed-coats have been removed after soaking in water. The seed-coats thus separated, warmed on a slide with a little saturated aqueous solution of choral hydrate and examined under the microscope, are seen to contain not more than an occasional crystal of calcium oxalate. The odour of the crushed seed is characteristic, though not very powerful; the taste is intensely bitter. The student should observe

(a) The colour of the seed and the satiny hairs,

(b) The straight cotyledons and narrow endosperm,

(c) The green reaction with sulphuric acid.

Constituents

The active constituent of strophanthus seeds is the glucoside strophanthin, of which they contain from 8 to 10 per cent. This substance, which is the source of the green reaction with sulphuric acid, is present in the endosperm, and frequently also in the cotyledons.

The seeds contain, in addition, about 30 per cent, of fixed oil, together with kombic acid, strophanthic acid, choline, and trigonelline. They yield about 4 per cent, of ash on incineration.

Strophanthin, C40H66O19, forms colourless, intensely bitter, very hygroscopic crystals freely soluble in water and dilute alcohol, sparingly soluble in absolute alcohol, and almost insoluble in ether or chloroform. Warmed with a dilute mineral acid it is hydrolysed into strophanthidin and a sugar (strophanthobiosemethylether). Strophanthidin is also crystalline, • but almost insoluble in water, and separates in crystalline form when strophanthin is hydrolysed.

It must, however, be observed that commercial strophanthin has been shown to vary considerably in toxicity and that the strophanthins prepared by different chemists from (as far as can be ascertained) the same variety of seed, viz. S. Kombe, have shown differences in the melting-point, in the hydrolysis, in the products yielded by the hydrolysis, and in other respects, so that they appeared to be different substances. It must, therefore, be admitted that the chemistry of this body has not yet been sufficiently investigated. Brown and Closson (1914) assert the presence of two strophanthins, a crystalline and an amorphous.

Strophanthic acid is an acid saponin (see p. 254).

Assay

The chemical methods that have been proposed for the assay of strophanthus seed depend upon the extraction of the glucoside, and conversion into strophanthidin which is weighed and from which the strophanthin present is calculated. They are, however, all unsatisfactory. The drug may be standardised by the bio-chemical method which consists in determining the quantity of tincture which, after dilution with an equal volume of water, will kill a normal frog of about 25 gm. weight within an hour when injected into the dorsal lymph sac. The amount required has been designated a ' unit.' The seeds or their preparations may be compared in activity by determining experimentally the quantity that represents a unit.

Uses

The action of strophanthus seed, which is due to the strophanthin they contain, resembles that of foxglove leaves. It raises the blood-pressure, is an efficient diuretic, and a powerful cardiac poison. It is not cumulative, and less liable than foxglove to produce gastro-intestinal irritation; hence is sometimes substituted for foxglove when this remedy has failed or disagreed.

Varieties

The official strophanthus seed often occurs in commerce mixed with or replaced by the seeds of other species of Strophanthus which resemble them more or less closely. The following are the most important.

S. hispidus, de Candolle. - The seeds of this species are exported from West Africa; they are smaller than the official, although similar in shape, are brownish in colour, bear scattered hairs, and give with sulphuric acid a green reaction. They contain strophanthin (h-strophanthin; see below) said to be closely allied to, if not identical with, the amorphous strophanthin of Kombe seed.

S. Courmontii, Sacleux. - This is an East African species; the seeds usually have a brownish tinge, but often closely resemble the official, from which, however, they may be distinguished by their rather smaller size, lanceolate shape, less bitter taste, red reaction with sulphuric acid, and abundant prismatic calcium oxalate crystals in the seed-coat. The active constituent is unknown. The seeds are said to be about one-fourth as active as Kombe seeds.

S. Nicholsoni, Holmes. - The seeds of this species are whitish and woolly, the hairs project beyond the apex of the seed, and the reaction with sulphuric acid is red; they have been exported from Natal. The active constituent is unknown.

S. gratus, Franchet. - The seeds are brown and glabrous; they give a red reaction with sulphuric acid, and contain the glucoside ouabain, which was first isolated from the wood of Acokanthera Schimperi, Oliver, and is much more toxic than strophanthin. For this glucoside the distinctive name of ' gr-strophanthin ' has been proposed (Thorns, 1900); its use has been advocated as a substitute for the official drug, as the seeds are readily obtainable and easily recognised.

S. Emini, Aschers. - The seeds are greyish green, but they contain cluster crystals of calcium oxalate, and give a red reaction with sulphuric acid. They contain the glucoside e-strophanthin (Thorns), which is distinct from the strophanthin of Kombe seed (k-strophanthin) and also from that of the seed of S. hispidus (h-strophanthin). Their action on the heart is said to be slight.