Lectins are sugar binding proteins (or glycoproteins) of non-immune origin with the ability to agglutinate cells and/or precipitate glyconjugates. Lectins can recognise specific sugars on cell surfaces resulting in cell agglutination. Lectin induced agglutination is inhibited by a specific sugar and this distinguishes lectins from mammalian antigens.

Lectins are present in almost every organism but are readily detectable in different parts of a large number of species of plants (see Appendices 32, 33, 34).

As the area of applications of lectins is very vast, only the aspects relevant to this book are discussed here. A more detailed picture is obtainable from the references cited here and the cross references therein.

Lectins can induce bacterial cell agglutination and inhibit their growth. This characteristic may be useful as an effective tool in the identification of pathogens and control infections caused by them. In addition, lectins can pre-empt the binding sites on bacterial cell surfaces preventing the binding of the pathogen to human skin, gut and other surfaces and membranes, which is an important event in pathogenesis.

Lectins, due to their specificity to cell surfaces, can be used to identify cell types, can be employed in the determination of human and animal blood groups and to immobilise human sperms through agglutination, or detect the onset of cancer.

Work in our laboratory has shown that lectins are present in several species of plants used in medicine (Shubha Rani, 1988, 1995; Sathyananda, 1989; Sharu Raj, 1990; Sangeetaa, 1994; Sathyanarayana Bhat, 1993; Sharon, 1994) and also in plants used as food (Sharu Raj, 1990), as detailed in Appendices 32, 33, and 34. Seeds, roots, leaves, bark and latex were tested for lectins against the erythrocytes of the four groups of human blood in the ABO system, and six animals. The agglutination tests were conducted with phosphate bufferred saline (PBS) extracts of plant tissues with and without polyvinyl pyrrolidone (PVP; used to remove compounds that interfere with cell agglutination), following standard procedures.

The problems associated with lectin assay and the precautions needed to be taken, were discussed by Kameswara Rao and Sangeetaa (1996). One point to be remembered is that, lectin assay should be based on a wider variety of cell types. The variation in the carbohydrate specificity of lectins and the variation in the surface carbohydrates of different cell types make it essential that lectin assay is not done using just one cell type like the human erythrocytes of a particular group or rabbit erythrocytes, when we would miss lectins of a different specificity. Data in Apppendix 34 amply demonstrate this point.

LECTINS IN MEDICINAL PLANTS

Our survey for lectins in parts of plants used as sources of drugs has demonstrated the presence of lectins in a large number of them (Appendices 32, 33, 34). The scope of utilising these lectins in different contexts was evaluated by the respective workers (Sathyanarayana Bhat, 1993; Sharon, 1994; Shubharani, 1995).

LECTINS IN FOOD PLANTS

A survey for the presence of lectins was made in 191 samples of the species used as food (Appendices 32, 33, 34). The implications for the presence of lectins in parts used as food, often in the raw condition, in terms of health and nutrition, were discussed by Sharu Raj (1990). An intervarietal difference in the distribution of lectins was evident in the case of rice and tomato.

LECTINS IN PLANTS USED IN DENTAL CARE

Extracts of a number of species of plants used as tooth brushes, chewing sticks and tooth powders were found to contain lectins (Appendices 10, 32, 33, 34). Plant extracts containing lectins agglutinate and inhibit growth of the oral bacteria (Shubha Rani, 1995) and so should help to check oral bacterial infections and to reduce the chance of development of plaque, the first step in the onset of caries. As lectins agglutinate human erythrocytes, they can be used as as haemostatic agents to control bleeding of gums and other oral parts (Shubharani, 1995).

SPERM AGGLUTINATING LECTINS

Sperm agglutinating lectins are of interest as they bind the sperms and immobilise them. This makes sperm agglutinating lectins useful as topical contraceptives.

The surface carbohydrates of the head and tail of human sperm are different. Consequently some lectins bind the sperms by the head (for example, lectin in the seed of Abrus precatorius) and some by the tail (as the lectin in the leaf of Pelargonium graveolens) while some lectins bind them both by the head and the tail (as the lectins in the bark of Anacardium occidentale, seed of Artocarpus heterophyllus, seed of Bauhinia racemosa, seed of Cardiospermum helicacabum, leaf of Clerodendrum serratum, and fruit rind of Punica granatum) (Sathyanarayana Bhat, 1993). Extracts of 494 species were tested of which 122 contain sperm agglutinating lectins (Appendix 34).