A comparative assessment of lactate dehydrogenase isozymes, LDHk and LDH5.

An apparently unique isozyme of lactate dehydrogenase has been reported associated with transformation by Kirsten sarcoma virus, which was also expressed in human cancer. This isozyme was designated LDHk (Anderson, G.R., and Kovacik, W.P., Jr., (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 3209-3213; Anderson, G. R., Kovacik, W. P., Jr., and Marotti, K. R. (1981) J. Biol. Chem. 256, 10583-10591). However, preparations of LDH5 from human placenta and from HeLa cells were later shown to exhibit some of the properties ascribed to LDHk9 and the identify of LDHk as a unique isozyme was questioned (Morin, M. E., and Hance, A. J., (1983) J. Biol. Chem. 258, 2864-2869). Saavadra and Anderson (Saavedra, R. A., and Anderson, G. R. (1983) Science (Wash. D.C.) 221, 291-292) refuted the arguments of Morin and Hance (Morin, M. E., and Hance, A. J. (1983) J. Biol. Chem. 258, 2864-2869) by claiming that commercial preparations of human placental LDH5 were contaminated with LDHk. Re-evaluation of the unique properties which distinguish LDHk from conventional LDH5 indicates that the two isozymes may not be different. Highly purified preparations of LDHk exhibit a single Mr = 34,000 polypeptide subunit on sodium dodecyl sulfate-acrylamide gels, yet retain activity detectable as both LDHk and LDH5. Attempts to separate LDHk and LDH5 by column chromatography or by continuous electrophoresis on a variety of solid support matrices were unsuccessful. Enzyme activity identified as LDHk in imidazole-borate-buffered gels migrating toward the cathode was detected as LDH5 activity on re-electrophoresis. LDH5 activity identified by electrophoretic migration toward the anode in Tris-glycine-buffered gels also recorded as LDHk when re-electrophoresed toward the cathode in imidazole-borate-buffered gels. Quantitative assays of enzyme activity recovered from the two-gel assay systems, as well as re-electrophoresis of isozyme-enriched preparations, indicated that cross-contamination of isozymes was not responsible for the results obtained.