Micro-complement fixation: a quantitative estimator of protein evolution.

The quantitative immunological technique of micro-complement fixation (MC'F) has been routinely used during the past decade to assess evolutionary relationships among living vertebrate species. The large data base that has been generated, along with the excellent correlations between immunologically measured genetic distances and paleontologically derived estimates of divergence times, have formed the basis for the albumin molecular clock. Immunological distance (ID) involves a logarithmic transformation of experimentally measured antibody concentrations. The justification for this transformation has rested entirely on empirical correlations. Consequently, several other transformations have been proposed as giving better fits to particular data sets. We derive, from first principles, the relationship between ID and the amino acid sequence replacements (AAR) between compared albumins. ID is shown to be a linear estimator of AAR. This ID-AAR relationship is based on a proposed process of antibody assortment and exclusion. We present experimental data confirming that such an antibody assortment-exclusion process occurs in MC'F. This process can explain both the high sensitivity and the quantitative phylogenetic nature of the MC'F assay. The assortment-exclusion process also predicts a divergence limit beyond which MC'F data no longer provide robust phylogenetic data.