Protein Metabolites

4.3.1 Medical Uses of the Anti-AIDS Drug, Trichosanthin

Trichosanthin is a protein produced primarily in the tuberous roots of the Chinese Cucumber plant (Trichosanthes kirilowii). The gene that is responsible for the synthesis of trichosanthin has been cloned and sequenced by Clontech Company in California. The Chinese cucumber has been used in oriental medicine for several thousand years to promote abortions. More recently, it has been demonstrated that trichosanthin selectively inhibits replication of HIV virus in vitro by inhibiting ribosomal protein synthesis and cellular reproduction (mitotic activity). However, we need to point out that extracts of Chinese cucumber are very toxic and should never be ingested unless under the supervision of a physician. Further, the use of this plant is contraindicated in pregnant women or those of child-bearing age. Trichosanthin can cause severe reactions in humans, including pulmonary and cerebral edema, cerebral hemorrhage, myo-cardial damage, seizures, fever, damage to blood cells, and even death in one AIDS patient.1516

4.3.2 Uses of Seed Storage Proteins in Human Nutrition

During grain-fill in cereals, proteins [albumin (water-soluble), prolamin (alcohol-soluble) globulin (salt-soluble), and glutelin (alkali-soluble)] are stored in the outermost layer of the starch-storing endosperm tissue.17 This is called the aleurone layer; it is located just inside the seed-coat or pericarp. Such proteins are stored in protein body organelles in the cells of this layer. In cereal grains such as rice (Oryza sativa), wheat (Triticum aestivum), barley (Hordeum vul-gare), oats (Avena sativa), and rye (Secale cereale), the percent dry weight of the grain that is made up of protein varies between 6 and 12%. If one eats white rice, one gets little or no protein except for a very small amount in the embryos (or germs). Why is this so? It is because white rice, during milling, has been polished to remove the brown aleurone layer that contains the bulk of the protein in the grain. If white rice is eaten, and one is a vegetarian, other protein sources must be found, such as whole grains of oats, wheat, rye, or barley; or of high protein seeds of soybean (Glycine max), teff (Eragrostis abyssinica), millets (Panicum spp., Pennisetum glaucum, Eleusine coracana), wild rice (Zizania aquatica), and grain amaranth (Amaranthus spp.).18 Some of these contain much higher amounts of protein, varying between 20% (millets, grain amaranth, and teff) and 45 to 48% (soybean).

4.3.3 Secretion of Enzymes that Digest Insects in the Leaves of Pitcher Plants

Pitcher plants are insectivorous plants that live in wetlands, such as bogs [Sarracenia spp., Darlingtonia californica (Figures 4.5, 4.6)] or on limbs of trees in

FIGURE 4.5 A native population of the cobra plant, Darlingtonia californica, near Coos Bay, Oregon Coast. (Photo by Dr. Larry Mellichamp, University of North Carolina at Charlotte.)

tropical rain forests as epiphytes (literally, on plants) (Nepenthes spp., Figures 4.7, 4.8). These environments are nitrogen-poor, and none of these plants form symbiotic relationships with bacteria or blue-green algae which fix atmospheric nitrogen. So, to obtain nitrogen, these plants have evolved various mechanisms to trap insects and to secrete hydrolytic enzymes (e.g., chitinases, proteases) that digest soft tissues of the insects, thereby releasing nitrogen-containing compounds in the form of amino acids that the pitcher plants can assimilate.

The insect-trapping mechanisms of the leaves ("pitchers") developed by these plants are ingenious.1920 They include the following:

• Emission of distinctive odors from the leaves that attract insects to the leaves

• Occurrence of an attractive color (red, due to anthocyanin pigments) that serves to lure the insect to the pitchers (as seen with Nepenthes, and even more so, with Sarracenia leaves)

• Provision of "ladders" along the outside of the pitcher that facilitate access of the insect to the mouth of the pitcher (as seen in Nepenthes spp.)19a

• Possession of translucent window-like areas in the hood over leaf pitchers in Darlingtonia californica that confuse and disorient the insect once it is inside the hood portion of the leaf

• Secretion of sugary nectar from glands located near the rim of the pitcher that provide a food source lure/reward for the insect that visits the leaf

FIGURE 4.6 Close-up view of the cobra plant, Darlingtonia californica, photographed at Gasquet, CA. (Photo by Dr. Larry Mellichamp, University of North Carolina at Charlotte.)

• The occurrence of a slippery, thick waxy cuticle on the inside of the pitcher; "as the insect struggles up the surface, their feet become coated with wax, which builds up until the victims seem to have acquired heavy clodlike boots"20

• The occurrence of downward-pointing hairs near the rim of the pitcher that prevent the insect from getting out of the pitcher, once trapped inside

• The occurrence of a pool of liquid (up to 1 l in some Nepenthes spp.) containing digestive enzymes that not only drowns the insect, but also results in their being partially digested

How many insects can a single pitcher trap per day? The answer, according to ecologists who have examined this question for Darlingtonia californica in the state of Oregon, is as many as 50 per day. That many could provide a lot of nitrogen to the plant!

FIGURE 4.7A The pitcher plant, Nepenthes, growing in the conservatory at the University of Michigan Matthaei Botanical Gardens. (Photo by Peter Kaufman.)

Of what concern and use are pitcher plants to humans? Are they good? Are they bad? What is their relevance to protein metabolites? They are indicator plant species of habitats (wetlands like fens, tropical ecosystems) that are basically nitrogen-poor. Because of their rarity in such habitats, they are placed on rare and endangered plant species lists. They are wonderful subjects to study and to photograph by people of all ages who visit such ecosytems or see them on display in conservatories.1911 They have relevance to protein metabolites in that these plants get most of their nitrogen for synthesis of proteins from the insects that they digest in their leaf-pitchers!

FIGURE 4.7B Close-up view of one of the pitchers (a modified leaf). (Photo by Peter Kaufman.)

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