Old Yellow Enzyme. The discovery of multiple isozymes and a family of related proteins.

Using fast protein liquid chromatography, we have separated native Old Yellow Enzyme from Brewer's Bottom Yeast into three distinct fractions. Two of these fractions are homodimeric forms of the enzyme while the third is the corresponding heterodimeric form. One of these homodimeric fractions is identical in every respect to OYE1, originally cloned from Brewer's Bottom Yeast (Saito, K., Thiele, D. J., Davio, M., Lockridge, O., and Massey, V. (1991) J. Biol. Chem. 266, 20720-20724). We have cloned, sequenced, and expressed a second Old Yellow Enzyme gene from Saccharomyces cerevisiae, showing close similarity, but not identity, with OYE1. Native Old Yellow Enzyme samples were also affinity-purified from a strain of S. cerevisiae and an OYE deletion mutant constructed from it. A total of at least seven isozymes of Old Yellow Enzyme have been discovered, each having slightly different characteristics ranging from surface charge to NADPH dehydrogenase activities with different electron acceptors, as well as N-terminal amino acid sequence. In addition, both recombinant enzymes showed considerable similarity to two proteins in the GenBank/EMBL data bank, a 60,000-dalton bile acid-inducible polypeptide in Eubacterium sp. (Mallonee, D. H., White, W. B., and Hylemon, P. B. (1990) J. Lipid Res. 172, 7011-7019) and a 72,000-dalton NADH oxidase in Thermoanaerobium brockii.