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On the Biochemical Differences between Adh Allozymes in Drosophila.

MCDONALD, ANDERSON and SANTOS (1980) reported apparent biochemical differences between the allozymes of alcohol dehydrogenase from Drosophila melanogaster and ascribed certain adaptive properties to the polymorphism on the basis of the specific nature of the observed biochemical differences. We believe that the authors’ approach to the study of biochemical variation is an important one and that functional characterization of allozymes can provide information about the evolutionary significance of enzyme polymorphism. However, their data do not provide evidence for co-enzyme (NAD)-alcohol interactions in enzyme affinity between the allozymes, and their adaptive interpretations for the maintenance of the polymorphism fail to take into account certain data that they reported. On the first point, MCDONALD, ANDERSON and SANTOS did not consider the catalytic mechanism of ADH in their analyses. They incorrectly claim that the intersections of the family of lines above or below the abscissa in their Figure 1 constitute evidence for either a positive or negative effect of binding one substrate upon the enzymes’ “affinity” for binding the other. They refer to these effects as “heterotrophic” [sic] or “cooperative-like,” interactions between the co-enzyme and alcohol substrates. This inappropriate use of terminology is the source of some confusion. Heterotropic binding phenomena customarily refer to interactions between dissimilar ligands that bind to topologically distinct sites on the enzyme (see MONOD, WYMAN and CHANGEUX 1965, pp. 88-89). An example of a heterotropic interaction is the effect of modulator binding at an allosteric site on the binding/catalysis of substrate(s) at a catalytic site elsewhere on the enzyme. However, co-enzyme binds, as in all dehydrogenases, at the same catalytic site as the other substrate (e.g., alcohol in ADH) ; therefore, there can be no heterotropic interactions, in the conventional use of this term. MCDONALD, ANDERSON and SANTOS used this terminology in an unorthodox sense to refer to interactions between alcohol and co-enzyme substrates in their respective affinities to ADH at the same catalytic site. Use of “heterotropic interaction” between substrates in this sense implies that either the alcohol or COenzyme can productively bind first to ADH, i.e., that ADH possesses some kind of random bisubstrate kinetic mechanism. While the authors’ Lineweaver-Burk plots (Figure 2) are certainly consistent with such a mechanism (CLELAND 1970; FROMM 1975), there are two reasons why these data are not evidence for interactions in this latter sense. First, the authors’ double-reciprocal plots are also consistent with a steady state, obligatorily sequential bisubstrate mechanism, where the substrates must bind in a specific order to achieve catalysis. This mechanism is thought to be