Preparation of Lectin---Gold Complexes

Determine the minimum amount of antibody required to stabilize the colloidal gold sol being used. The colloidal suspension should be adjusted, if needed, with 0.1 M K2CO3 or NaOH to pH 8—9. Measure the pH of the sol using a gel-filled electrode. Determining the stabilization amount of antibody can be done according to the method described in Section 1. 

To remove excess IgG, centrifuge the preparation at a minimum of 50,000 g for 30 min to several hours (4°C), depending on the size of the particles and the amount of solution. Discard the supernatant and resuspend the antibody—gold pellet in 0.01 M sodium phosphate, pH 7.4, containing 0.25% BSA (or 1% PEG, as desired). 

Lectins, or proteins with specific binding sites for carbohydrates, can be used as targeting molecules to locahze particular glycoconjugates such as glycoproteins or glycolipids on ceil surfaces (Fig. 373). Labeled with gold particles, lectins are important probes for detection of cell surface components and intracellular receptors and in immunological or biochemical assay procedures (Bog-Hansen et al, 1978 Kimura et al., 1979 Nicolson, 1978 Roth, 1983 Benhamou et al., 1988 Nakajima et al., 1988). 

The following generalized protocol is an adaptation for the labeling of 15-nm gold particles with Aplysia gonad lectin, as described by Benhamou et al. (1988). Each lectin-gold preparation will have its own unique pH optimum and ratio of lectin-to-gold for the absorption process. 

After 1 minute, add 250pi of 1 percent polyethylene glycol (MW 20,000) per 10ml of gold sol used. The PEG helps to stabilize further the sol against aggregation. 

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Benhamou N (1989) Preparation and application of lectin-gold complexes. In Hayat MA (ed) Colloidal gold, vol 1. Academic Press, New York, NY, pp 96-145... 

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