Induction of serotonin secretion by cross-linking of surface receptors of a derivative of wheat germ agglutinin on human platelets

Degranulation of polymorphonuclear leukocytes is induced by multivalent cross-linking of wheat germ agglutinin binding site(s) on cell membrane

Polymorphonuclear leukocyte (PMN) surface membrane glycoproteins very likely are involved in the phenomenon of stimulus-response coupling. Previously, we have shown that subagglutinating concentrations of the plant lectin, wheat germ agglutinin (WGA) specifically and irreversibly inhibited N-formyl-methionyl-leucyl-phenyl-alanine (FMLP)-mediated PMN chemotaxis. WGA did not affect the binding of [3H]FMLP to its receptor on the PMN plasma membrane. We have examined the possibility that cross-linking of WGA binding sites may elicit PMN degranulation. We have found that multivalent, but not bivalent, cross-linking of WGA bound to PMNs elicits release of lysosomal constituents. This phenomenon was specific for WGA since it did not occur when concanavalin A (Con A), instead of WGA, was used. It is intriguing to speculate that WGA may attach to a "physiologic" receptor for FMLP on the PMN membrane and that redistribution (cross-linking) of this receptor might be an early event in the activation of PMNs by FMLP.

A novel thrombin enhancement factor in human plasma

A protein has been isolated from human plasma by gel filtration followed by affinity chromatography with a derivative of wheat germ agglutinin and ion exchange chromatography. This protein showed one peak in high performance liquid chromatography but in gel electrophoresis, in the presence of sodium dodecyl sulfate and beta-mercaptoethanol, revealed two major components of 74 kDa and 55 kDa. These results indicate that the protein probably exists as a complex of the two polypeptides. This protein complex enhanced platelet aggregation by thrombin while aggregation induced by ADP was not significantly affected. Similarly, the rate of thrombin action on fibrinogen and N-benzoylarginine ethyl ester as measured in a spectrophotometer was increased in the presence of this plasma protein. These results suggest the presence of a protein complex in human plasma which can directly interact with thrombin and enhance its reactivity.

Preparation and some properties of a derivative of wheat germ agglutinin with altered biological activity

An inactive derivative of wheat germ agglutinin, which is a strong activator of blood platelets, was prepared by selective chemical modification of the lectin with cyanogen bromide at acid pH. The derivative was then used as a probe to learn about the initial events in platelet stimulation by physiological agents. Amino acid analysis of the modified lectin confirmed specific cleavage of a methionine residue. Gel filtration studies indicated a molecular weight for the lectin derivative similar to the unmodified lectin. In gel electrophoresis in the presence of sodium dodecyl sulfate, reduced samples of the derivative showed two bands and the main component migrated slightly faster than the native lectin. The derivative retained the capacity to precipitate an antibody to the lectin although at least one of the antigenic sites was lost due to chemical modification. The derivative did not compete with the unmodified lectin for binding to platelets. Unlike the parent lectin, the derivative did not aggregate platelets even at a ten fold higher concentration. Under similar conditions, there were about 1.0 X 10(5) binding sites/platelet for the lectin derivative with an apparent dissociation constant of 1.7 microM compared to 5 X 10(5) sites/cell and a dissociation constant of 0.4 microM for the native lectin. Overnight incubation of platelets or red cells with the derivative in microtiter plates showed about 2-5% agglutinating activity for the derivative compared to the unmodified lectin. Incubation of platelets with the lectin derivative inhibited platelet aggregation by thrombin while aggregation induced by a number of other agents was not significantly affected. This inhibitory effect of the lectin derivative on thrombin-induced platelet aggregation could be readily reversed with GlcNAc. The lectin derivative may be a useful tool to explore the structure-function relationship of cell surface components.

Inhibition of thrombin binding and serotonin secretion from platelets by a 74,000 dalton protein

A 74,000 dalton protein has been isolated from solubilized human platelet membranes by affinity chromatography with a nonagglutinating derivative of wheat germ agglutinin. The protein as well as the lectin derivative blocked platelet aggregation by thrombin while aggregation induced by several other agents was unchanged. This protein inhibited the binding of thrombin to platelets in a competitive manner. The secretion of serotonin from platelets by thrombin was also blocked by the protein. These effects of the protein were observed when platelets were suspended in either plasma or buffer. This protein may have functional significance in the activation of platelets by thrombin.