Enzyme specificity: "pseudopolymorphism" of lactate dehydrogenase in Drosophila melanogaster

An electrophoretic variant in the LDH (L-lactate:NAD oxidoreductase, E.C.1.1.1.27) of Drosophila melanogaster was observed on starch (or polyacrylamide) gels. This variant was found to exhibit an identical isozymic pattern (three isozymes with a decreasing staining density) on starch gel and map position as the Adh locus. On the other hand, anodal polyacrylamide gel electrophoresis in crude extracts has shown LDH to consist of nine bands and ADH of four bands. We have shown than ADH (Alcohol: NAD oxidoreductase, E.C.1.1.1.1) also oxidizes L(+)-lactate or D(-)-lactate with the NAD, while LDH oxidizes ethanol. By using various genetic and biochemical techniques, we have shown that the observed Ldh electrophoretic variant was not a real one and could be attributed to the presence of ADH. We have called this phenomenon "pseudopolymorphism," and the problem of enzyme specificity has been examined. The appearance of a band in an assay using lactic acid as a substrate is not sufficient evidence for the presence of LDH. Hence, caution is called for before characterizing an electrophoretic band on a gel as being equivalent to the presence of a genetic locus. Out of the nine electrophoretic zones of activity observed on polyacrylamide gel (or out of the six previously observed) using crude extract, only two (one major and one minor) belong to LDH, as revealed by purified enzyme preparations. Furthermore, purified LDH exhibits activity in two bands on starch gel (out of three observed in crude extracts), which appear in different positions as compared with those of ADH. Finally, one band which responds to the presence of D(-)-lactate but not to L(+)-lactate has been revealed.