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)