Inhibition of lymphocyte 5′-nucleotidase by lectins: effects of lectin specificityand cross-linking ability

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Abstract
5′-Nucleotidase, an integral glycoprotein enzyme of the lymphocyte plasma membrane, is inhibited cooperatively by the lectin concanavalin A. Because divalent succinyl-concanavalin A is a poor enzyme inhibitor, both binding and lectin-induced cross-linking of 5′-nucleotidase may be necessary for inhibition. Succinyl-concanavalin A does not compete with concanavalin A for binding to the enzyme; however, maleyl-concanavalin A, another poor inhibitor, competes effectively with the parent lectin. Thus, maleyl-concanavalin A binds to the same site as concanavalin A but causes little inhibition, whereas succinyl-concanavalin A does not bind to this site. The monovalent lectin from Ricinus communis (RCA-60) is a more effective enzyme inhibitor than the related divalent lectin (RCA-120), and inactivation of the second low-affinity sugar binding site on RCA-60 does not abolish inhibition, suggesting that multivalent cross-linking is not required for 5′-nucleotidase inhibition. Peanut and wheat germ agglutinins do not inhibit the enzyme, whereas lectins from lentil, pea, soybean, Griffonia simplicifolia, and Phaseolus vulgaris inhibit 5′-nucleotidase with various degrees of effectiveness. The only lectin showing strong positive cooperativity in its interaction with 5′-nucleotidase is concanavalin A.