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Fructose-l,6-bisphosphate aldolase from Drosophila melanogaster, inactivated by reducing the Schiff base between the active-site lysyl residue and the substrate, formed enzymatically active hybrids with native C aldolase from calf brain. The fact that hybridisation takes place indicates that blockage of the active site of Drosophila aldolase with reduced substrate does not significantly change the quaternary structure of the enzyme. Furthermore our results show that each subunit in the tetrameric structure of aldolase makes an independent contribution to enzymatic activity.Several laboratories have reported that fructose-1,6-bisphosphate aldolases from certain organs of a number of vertebrates (class I aldolases) often exist in a five-membered set of isoenzymes [I -3 j. These isoenzymes may be separated electrophoretically due to differencies in their net electrical charge. By various techniques it has been shown, that class I aldolases are tetramers [4-61 and that the five-membered set can be explained by the random interaction of the subunits of two homomeric aldolases to form tetrameric molecules : two homomeric species (A, and B,) and three hybrids (A,B, A,B,, AB,). The formation of hybrids appears t o be due to similar conformation of the enzyme monomeric units [7,8j. Five-membered hybrid sets can also be produced in vitro by the combination of two parental aldolases. I n an earlier communication [9] we described the formation of interspecific hybrids between the aldolase from Drosophila melanogaster (also a class I aldolase) and the C aldolase from calf brain. Since hybrid formation occurred, it was deduced that fructose-l,6-bisphosphate aldolases from vertebrates and invertebrates are homologous enzymes, i.e. genetically related.I n addition to hybridisation experiments on naturally occurring variants of aldolases, we hoped to get more information about subunit function by hybridisation of a native enzyme with a modified form. These experiments were carried out with native aldolase from calf brain and modified aldolase from Drosophila. The

Volumen XXXIX, Fasciculus 111 (1956) -KO. 104. 863 l) Verwendete Abkurzungen : Coil = Coenzym A, ATP = Adenosintriphosphat, ADP == Adenosindiphosphat, AMP = Adenosinmonophosphat, PP = Pyrophosphat, DPN = Diphosphopyridinnucleotid, DPNH = reduziertes Diphosphopyridinnucleotid, SDE = Anthranildiessigsauro, Komplexon = Athylendiamin-tctraessigsaure, Glycolkomplexon = Glycolatherdiamin-tetraessigsilure.

Larvae of Drosophila melanogaster (wild type and mutant Sepia) were fed with labelled glycine, formate and glucose and the specific radioactivity in uric acid and pterines of the flies was measured. These experiments showed that carbon atoms of the glucose are specifically incorporated into the pterines but not in the purines. Possible pathways are discussed.

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