Using a radioimmunoassay (RIA) based on the Farr technique with radioactively labeled 3-H-DNA for quantitative measurements of anti-DNA antibodies in sera of patients with systemic lupus erythematosus (SLE), the influence of molecular weight of DNA (ranging from 0.1 times 10-6 to 22.0 times 10-6 daltons) on binding and precipitation in this system has been investigated. Comparing our results with mathematical models it follows that one antibody molecule is fixed on the average to a statistical DNA segment of 2 times 10-6 to 4 times 10-6 daltons. Furthermore binding capacity of the DNA was found to be independent of the molecular weight, as demonstrated in a double label experiment using 14-C and 3-H-labeled DNA of different size. However, the amount of radioactivity precipitated was found to depend on the molecular weight of the labeled DNA following a non-linear function. It was calculated that a minimal ratio of fixed antibody molecules per a certain size of DNA was necessary for precipitation. The mathematical treatment of the observed non-linear precipitation dependence will be discussed using various statistical models. Our results indicate that the quantitative measurements of anti-DNA antibodies with the Farr technique e.g. for diagnosis and control of SLE in clinical immunology is highly dependent on the molecular weight of the labeled DNA used in the assay system and reliable results are only obtained with DNA of a sufficiently high molecular weight.

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The extracellularly localized, galactose‐specific lectin from the sponge Geodia cydonium binds at one class of sites, 40 mol Ca2+/mol lectin with an association constant (Ka) of 0.3 × 106 M−1. Stoichiometric calculations reveal that in the extracellular milieu 22 mol Ca2+ (maximum) are complexed per mol lectin. Binding of Ca2‐ to the lectin increases its apparent Mr from 44000 to 56000 (electrophoretic determination) or from 36500 to 53500 (high‐pressure liquid gel chromatographical determination); the S20,w increases from 4.3 S to 4.5 S if Ca2‐ is added to the lectin. In the presence of Ca2+ the lectin undergoes a conformational change perhaps by expanding the carbohydrate side chains which are terminated by galactose. Subsequently the lectin molecules polymerize to large three‐dimensional clumps (diameter up to 8 μm). Turbidimetric studies reveal an inhibition of the lectin polymerization by lactose. The Ka of the lectin‐lectin polymerization rises from 0.9 × 106 M−1 to 14.0 × 106 M−1 after increasing the Ca2+ concentration (from 1 μM to 100 μM). Parallel with this increase in affinity, the Ka value of the lectin‐aggregation factor binding drops from 41.2 × 106 M−1 (1 μM Ca2+) to 1.3 × 106 M−1 (100 μM Ca2+). In the absence of Ca2+, the Geodia lectin forms 1–10‐μm two‐dimensional sheets in the presence of homologous glycoconjugates. Cell binding experiments with polyacrylamide gels, containing covalently bound galactose, show that both homologous (Geodia cydonium) and heterologous cells (L 5178y) bind with a higher affinity to the lectin‐polymer matrix than to the lectin‐monomer one. These data suggest that lectin‐polymer structures, together with lectin‐glycoconjugate associates, are components of the cell‐substrate adhesion system(s) of sponges in vivo.

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