Describing an S-shaped curve, for example, the shape of enzyme-kinetic curves for enzymes displaying positive homotropic cooperativity.
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where E is the pharmacological effect, [E.sub.max] is the calculated maximum effect, [EC.sub.50] is the drug concentration in the effect compartment yielding half of the maximal effect, and n is the sigmoidicity factor, an exponent describing the number of drug molecules that combine with each receptor molecule.
This method involves plotting dose-effect curves, for each agent and their combination, using the median-effect equation: [f.sub.a]/[f.sub.u] = (D/Dm)m, where D is the dose of the drug, Dm is the dose required for a 50% effect (equivalent to IC50), [f.sub.a] and [f.sub.u] are the affected and unaffected fractions, respectively ([f.sub.a] = 1 - [f.sub.u]), and m is the exponent signifying the sigmoidicity of the dose-effect curve.
More and better measurements of handling times and energy requirements in the field would be required to draw a definite conclusion about how the "sigmoidicity" of the functional response affects the likelihood of mutualism.
when m [not equal to] 1, in which D is the dosage of the drug, [D.sub.m] is the median-effect dosage signifying the potency, determined from the x-intercept of the median-effect plot: [f.sub.a] is the fraction affected by the dose; [f.sub.u] is the fraction unaffected ([f.sub.u] = 1 - [f.sub.a]); and m is an exponent that signifies the sigmoidicity (shape) of the dose-effect curve, which is determined by the slope of the median-effect plot.
The advantage of this method is that it takes into account not only the potency (median effect dose values [[D.sub.m]] or drug concentration at 50% neutralization [[EC.sub.50]]), but also the shape (sigmoidicity) of the dose-effect curve, based on the median effect equation of Chou.