Differential binding of the insecticidal lectin GNA to human blood cells

Brian Fenton, Kiri Stanley, Steven Fenton, Caroline Bolton-Smith / The Lancet V.354, N.9187 16oct99

Evidence of snowdrop lectin binding to human white cells supports the need for greater understanding of the possible health consequences of incorporating plant lectins into the food chain.

There is interest in the possible use of lectins to protect food plants from attack by insects. Many of these carbohydrate-binding proteins agglutinate vertebrate red blood cells. The lectin peanut agglutinin (PNA) also binds to the Thomsen-Friedenreich antigen on the surfaces of some human colon cells. After eating peanuts, PNA has been detected in the blood after 1 hour¹ and individuals who express this antigen have increased rates of colonic cell division, with possible health implications.² PNA does not appear to be under consideration in an insecticidal role. Galanthus nivalis (snowdrop) agglutinin (GNA) is however under consideration and transgenic plants expressing GNA have been constructed.³ GNA recognises terminal 1-3-linked mannosyl residues. The distribution, abundance, and microheterogeneity of this structure on human glycoproteins is largely unknown, particularly for membrane-bound receptor proteins. Although the conventional view is that mammalian intestinal cells possess no free mannose residues, and therefore cannot bind GNA, there have been reports of dietary effects of GNA in rats, including hypertrophy of the small intestine.⁴ GNA is highly resistant to digestion, and tests on human cells suggest that GNA can cause increase in mitosis. Although mitogenic stimulation requires both lectin binding and activation of appropriate cell-surface receptors, poorly mitogenic lectins can act in synergy with other compounds to increase mitotic indices. In addition, GNA could react with other receptors and block their normal function in at least some tissues and in at least some individuals.

We examined proteins isolated from human buffy coats and red blood cells taken on three different occasions from six healthy individuals (aged more than 60 years) for their reactivity with GNA. Sodium dodecyl sulphate polyacrylamide gel electrophoresis and western blot procedures were used to transfer proteins onto a nitrocellulose support matrix.⁵ This procedure was followed by detection with commercially available biotinylated lectins and alkaline phosphatase linked to streptavidin. Our results show that human white blood cells have many proteins that strongly bind to GNA (figure). This binding was partially inhibited in the presence of mannose. Red blood cells showed little reactivity with GNA which was consistent with the lack of agglutinating activity of GNA for human erythrocytes and also indicated that blood group variation was not an issue

Lane 1 molecular weight markers; lanes 2­7 individuals

1­6 buffy coats; lanes 8­13 RBCs; lane 14 ribonuclease A (not mannosylated; negative control); lane 15 ribonuclease B (mannosylated; positive control)

Equal volumes of buffy coat and RBC were used. Note the intense staining of glycoproteins in the buffy coat layer but weak staining of RBC glycoproteins. Arrows indicate possible polymorphic glycoproteins.

These data strongly suggest that human glycosylation pathways in white cells are capable of synthesising substantial quantities of terminal mannose moieties that interact with GNA. Furthermore the reaction appears to vary (indicated by arrows). This work highlights the need for a much greater understanding of the interactions between plant lectins and human glycoproteins before they can be safely incorporated into the food chain.

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Scottish Crop Research Institute, Invergowrie, Dundee, UK (B Fenton PhD, K Stanley BSc), and Nutrition Research Group, Cardiovascular Epidemiology Unit, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK (S Fenton MSc, C Bolton-Smith PhD)

 

 

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