**6. Plant lectin**

Lectins have been found in a wide variety of species almost every major taxonomical classification of flowering plants (Allen and Brilliantine 1969; and Mialonier et al., 1973). Many plants and their individual tissues have been routinely screened for lectins by measuring the ability of their extracts to agglutinate erythrocytes. Although this hemagglutination assay has been of great value in detecting lectins, it is at best semiquantitative; it will not detect inactive or monovalent lectin, nor will it provide accurate estimates of lectin if an endogenous receptor for that lectin is present in the extract. The assay can at times yield false positive results because of nonspecific hemagglutination caused by lipids or by polyphenols such as tannins that are often abundant in plant tissues. It is therefore advisable to verify positive hemagglutination data by inhibiting the activity with specific sugars or by isolating the lectin (Tsivion and Sharon, 1981).

The carbohydrate specificities and structures of lectins from a large variety of plants have been studied in considerable detail. In general, lectins from plants within particular taxonomical groups have distinctive properties that distinguish them from lectins of less closely related plants. It is important to note that the lectins used in these comparisons represent the most abundant and therefore most intensively studied lectins in the plants of these families. These lectins are not all derived from homologous tissues. These differences in origin must be remembered in interpreting these comparisons since, as is discussed below, it is possible that different tissues within the same plant may contain different lectins. This reservation does not apply to comparisons of lectins obtained from homologous tissues of plants within the same family. Homologies within two of these families, the Graminaceae and Leguminoseae, are discussed in further detail below.

Graminaceae: The lectin from monocotyledonous plants is the wheat germ agglutinin, which is a 36,000 molecular weight dimer of identical protein subunits linked by interchain disulfide bonds (Nagata and Burger, 1974; and Rice and Etzler, 1974). The complete amino acid sequence of this lectin has recently been determined (Wright et al., 1984). This lectin has a specificity for oligomers of β (1-4)-*N*-acetyl-o-glucosamine (Allen et al., 1973). Lectins with similar specificities and molecular properties have been isolated from rye (Peumans et al., 1982b) and barley embryos (Mishkind et al.,1983; Peumans et al., 1982b). Indeed, these lectins are so similar that they can undergo subunit exchange to form heterodimers (Peumans et al., 1982a).

Leguminoseae: The seeds of legumes are particularly rich in lectins, and many of these lectins have been characterized extensively (Goldstein and Hayes 1978; Lis and Sharon 1986). As this review was prepared, the complete amino acid sequences of Concanavalin A (Edelman et al. 1972), favin (Cunningham et al., 1979), and lectins from lentil (Foriers et al., 1981), sainfoin (Kouchalakos et al., 1984), *Phaseolus vulgaris* (Hoffman et al., 1982), soybean (Hemperly et al., 1983), and pea (Higgins et al., 1983) have been determined. In addition, the NH2 terminal amino acid sequences of at least 15 other legume lectins are available. Comparisons of these sequences have shown extensive homologies, particularly among those lectins from plants within the same tribes. It is clear that these lectins have been conserved during evolution of the legumes and that the homologies in their NH2 terminal amino acid sequences reflect the taxonomical relationships of the plants in this family (Foriers et al., 1977; and Foriers et al., 1979).
